Upgrade React Native from v0.59 to v0.60.

Using the RN Upgrade Helper, a web app showing the diff from the
release/0.59.10 and the release/0.60.6 branches of the
`react-native-community/rn-diff-purge` repo, at
https://react-native-community.github.io/upgrade-helper/?from=0.59.10&to=0.60.6.

In this commit:

- Upgrade `react-native`, `react`, and `flow-bin` following the
  templates

- Upgrade `react-native-webview` to satisfy peer dependencies

- Adapt our Podfile to RN v0.60's new layout of pods, following the
  templates

- Upgrade `react-native-sound` and `rn-fetch-blob` to versions with
  podspecs compatible with the new pod layout in RN v0.60

A lot of work has already been done toward the upgrade:

- most of the fixes necessary to adapt to Flow 0.98, without
  upgrading Flow yet (zulip#3827). We do the upgrade in this commit, with
  warnings temporarily suppressed, for smaller commits.

- updating to AndroidX (zulip#3852)

- migrating to using CocoaPods at all (zulip#3987)

- We ignore or have already handled several changes, e.g., to the
  Xcode and Gradle configs (see comment at zulip#3548).

Note: The following warning appears when running Metro and will be
fixed with follow-up work (e.g., zulip#4118):

```
warn The following packages use deprecated "rnpm" config that will
stop working from next release:
  - react-native-orientation:
    https://github.com/yamill/react-native-orientation#readme
  - rn-fetch-blob: https://npmjs.com/package/rn-fetch-blob
```

----- Platform-agnostic --------------------------------------------

We upgrade `react-native-webview` in this commit because versions
before 7.x aren't compatible with RN v0.60, and versions 7.x aren't
compatible with RN v0.59 (judging by peer dependency warnings). We
take the latest 7.x version, 7.6.0.

Nicely, this gets us the changes from one of our PRs, released in
7.0.3; see 1982f3f and its reversion in the commit that followed,
bbfac73.

There's just one declared breaking change [1] in 7.x.x:

- UIWebView removed (7.0.1).

This prompted the `scalesPageToFit` prop to be removed, but we don't
use it. The `useWebKit` prop was also removed because it doesn't
make sense for it to be anything but `true` now. So, remove our use
of it.

Also run `yarn yarn-deduplicate && yarn`, as prompted by
`tools/test deps`.

[1]: https://github.com/react-native-community/react-native-webview#versioning

----- Android ------------------------------------------------------

There are no updates on the Android side that must happen atomically
with the RN upgrade.

----- iOS ----------------------------------------------------------

facebook/react-native@2321b3fd7 appears to be the only cause of iOS
changes that must happen atomically with the RN v0.60 upgrade.

In that commit, all the "subspecs" that were nested under the
"React" pod are un-nested so they each become their own pod, with
many of these living in their own directory under
node_modules/react-native/Libraries [1].

In our own code, this means changing our Podfile to refer to all
these new pods, instead of the "React" pod's subspecs. So, do.

Our Podfile has a list of "Pods we need that depend on React
Native". We must also be sure all the dependencies in this list
adapt to facebook/react-native@2321b3fd7.

The syntax for pointing explicitly to a subspec is a slash, as in
"React/Core" [2]. Knowing this, we check that list for pods that
explicitly depended on those subspecs, with "React/[...]":

```
grep -Rl dependency.*React/ --include=\*.podspec \
  --exclude="node_modules/react-native/*" node_modules
```

There are two, and they both have new versions that adapt to the new
accepted shape:

- `zmxv/react-native-sound@2f2c25a69`: "React/Core" -> "React"
- `joltup/rn-fetch-blob`, `01f10cb10^..0f6c3e3cc`: "React/Core" -> "React-Core"

So, take these new versions, and job done.

[1]: They do still live in node_modules. It's possible for pods to
     be hosted online and downloaded on `pod install`, npm-style.
     For an example, see the 'Toast' dependency in
     node_modules/react-native-simple-toast/react-native-simple-toast.podspec.
     But that's not what's happening here, yet.
     facebook/react-native@2321b3fd7 hints that this will be the way
     of the future for React Native, "to make the experience nicer
     for library consumers".

[2]: https://guides.cocoapods.org/syntax/podspec.html#subspec

Fixes: zulip#3548

.flowconfig

@@ -134,4 +134,4 @@ suppress_comment=\\(.\\|\n\\)*\\$FlowExpectedError
 suppress_comment=\\(.\\|\n\\)*\\$FlowMigrationFudge
 
 [version]
-^0.92.0
+^0.98.0

ios/Podfile

@@ -4,29 +4,28 @@ platform :ios, '10.3'
 
 require_relative '../node_modules/react-native-unimodules/cocoapods.rb'
 
-# This list will need to change for RN >= v0.60. If you're changing it
-# for any other reason, first look at the example Podfile for v0.59 at
-# https://github.com/facebook/react-native-website/blob/ded79d2cf/docs/integration-with-existing-apps.md#configuring-cocoapods-dependencies.
 target 'ZulipMobile' do
-  pod 'React', :path => '../node_modules/react-native', :subspecs => [
-    'Core',
-    'CxxBridge',
-    'DevSupport',
-    'RCTText',
-    'RCTNetwork',
-    'RCTWebSocket',
-    'RCTAnimation',
-    'RCTImage',
-    'RCTPushNotification',
-    'ART',
-    'RCTAnimation',
-    'RCTCameraRoll',
-    'RCTActionSheet',
-    'RCTGeolocation',
-    'RCTLinkingIOS', # RCTLinking?
-    'RCTSettings',
-    'RCTVibration',
-  ]
+  # Pods from React Native
+  pod 'React', :path => '../node_modules/react-native/'
+  pod 'React-Core', :path => '../node_modules/react-native/React'
+  pod 'React-DevSupport', :path => '../node_modules/react-native/React'
+  pod 'React-RCTActionSheet', :path => '../node_modules/react-native/Libraries/ActionSheetIOS'
+  pod 'React-RCTAnimation', :path => '../node_modules/react-native/Libraries/NativeAnimation'
+  pod 'React-RCTBlob', :path => '../node_modules/react-native/Libraries/Blob'
+  pod 'React-RCTImage', :path => '../node_modules/react-native/Libraries/Image'
+  pod 'React-RCTLinking', :path => '../node_modules/react-native/Libraries/LinkingIOS'
+  pod 'React-RCTNetwork', :path => '../node_modules/react-native/Libraries/Network'
+  pod 'React-RCTSettings', :path => '../node_modules/react-native/Libraries/Settings'
+  pod 'React-RCTText', :path => '../node_modules/react-native/Libraries/Text'
+  pod 'React-RCTVibration', :path => '../node_modules/react-native/Libraries/Vibration'
+  pod 'React-RCTWebSocket', :path => '../node_modules/react-native/Libraries/WebSocket'
+  pod 'React-cxxreact', :path => '../node_modules/react-native/ReactCommon/cxxreact'
+  pod 'React-jsi', :path => '../node_modules/react-native/ReactCommon/jsi'
+  pod 'React-jsiexecutor', :path => '../node_modules/react-native/ReactCommon/jsiexecutor'
+  pod 'React-jsinspector', :path => '../node_modules/react-native/ReactCommon/jsinspector'
+  # This is deprecated upstream; removing it is #4115. See
+  # https://reactnative.dev/docs/pushnotificationios.html.
+  pod 'React-RCTPushNotification', :path => '../node_modules/react-native/Libraries/PushNotificationIOS'
   pod 'yoga', :path => '../node_modules/react-native/ReactCommon/yoga'
   pod 'DoubleConversion', :podspec => '../node_modules/react-native/third-party-podspecs/DoubleConversion.podspec'
   pod 'glog', :podspec => '../node_modules/react-native/third-party-podspecs/glog.podspec'