UL&S: VoiceOver support for site address flow

[mindgraffiti]

...and UnifiedSignUpViewController

Ref. #14009
Ref. wordpress-mobile/WordPressAuthenticator-iOS#358
Auth PR: wordpress-mobile/WordPressAuthenticator-iOS#361

This PR updates three views to support VoiceOver:

• SiteAddressViewController
• SiteCredentialsViewController
• UnifiedSignUpViewController

And fixes bugs in the error handling that were only apparent after VoiceOver testing.

To test the Site Address flow

1. Enable the .unifiedSiteAddress feature flag
2. rake dependencies
3. Build and run on a device
4. Hey Siri turn on VoiceOver
5. Log In > Or by entering your site address.
6. It should read: "Site Address. Textfield. Is editing. example.com. Character mode. Insertion point at start."
7. (You can hey siring turn off VoiceOver so you can type in the address URL faster). Enter an invalid site, such as: fb.com (turn VoiceOver on again). Continue. Expected: the VoiceOver focus changes and reads the error message.
8. Enter a valid site. Select the site URL field again. It should change the reading now that a URL is entered. Tap Continue.
9. "Username. Textfield. Is editing. username. Character mode. Insertion point at start." Enter a username and tap the textfield. It will now read out "Username. Textfield. . Character mode. Insertion point at end." Tap Next on the keyboard to move to the password field.
10. "Password. Secure textfield. Is editing. password. Character mode. Insertion point at start." Feel free to toggle the show / hide password button and hear the state read back to you.

To test the first Unified Sign Up screen

1. Enable the .unifiedSignup feature flag
2. Build and run on a device
3. Sign up with WordPress.com > Sign up with Email.
4. It should read: "Back button." and stop. This is expected behavior for navigating a screen with no textfields. Two-finger swipe down to listen to all of the UI. It should end on "send link by email".

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[Gio2018]

Worked as expected on iPhone X device with iOS 13.6

[diegoreymendez]

While testing this I found what seems to be an issue when the user and password screen is shown (if VoiceOver is enabled when it comes up). The voice starts describing the title of this screen, and is interrupted when the focus is moved into the username field. I think we should either let it read the full screen description somehow, or prevent it from being read to begin with.

Here's a video showing the issue: https://cloudup.com/cw1-6dk7Pgd

Aside from the reported issue both tests run as described.

[mindgraffiti]

@diegoreymendez

the voice is interrupted

Darn. I thought I had fixed that issue. Thanks for testing, I'll look into a better solution.

[mindgraffiti]

Moving to milestone 15.6 in case I can't fix the podspec errors in Auth by EOD.

[mindgraffiti]

This PR will be merged only after 15.5 is cut. 🌻