Add basic auth support

[Pharb]

This adds support for request authentication as requested in #89 and #91 using https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/onAuthRequired.

A small workaround was required to support Chrome, see mozilla/webextension-polyfill#91.

Some manual test cases:

• Opening multiple basic auth sessions (e.g. with https://httpbin.org/basic-auth/user/password1, https://httpbin.org/basic-auth/user/password2) should only open a gopassbridge popup for the first auth request and the native browser auth dialog for any other auth requests, as long as another gopassbridge popup is still open. This is done to simplify the implementation.
• Disabling the basic-auth setting in options should show the native browser auth dialog when disabled and switching back to gopassbridge for the next auth request when reenabled.
• Closing the gopassbridge auth popup window without selecting any credentials should open the browser native auth dialog.
• Selecting invalid credentials in gopassbridge should reopen a new gopassbridge popup window (this is the same behaviour as the native browser auth dialog)
• Any errors in the native messaging between the popup and gopass should keep the popup open and show the error messages, until the window is either closed or some credentials are selected.

[martinhoefling]

Looks quite good in general to me. Some tests probably need to be written/adapted but it's far less complex than the passff implementation.

[martinhoefling]

Also tested the functionality with some basic auth dialogs on ffx and it works fine.

[codecov]

Codecov Report

Merging #99 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #99   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines         469    526   +57     
  Branches       79     91   +12     
=====================================
+ Hits          469    526   +57

Impacted Files	Coverage Δ
web-extension/generic.js	100% <100%> (ø)	⬆️
web-extension/gopassbridge.js	100% <100%> (ø)	⬆️
web-extension/create.js	100% <100%> (ø)	⬆️
web-extension/details.js	100% <100%> (ø)	⬆️
web-extension/background.js	100% <100%> (ø)	⬆️
web-extension/search.js	100% <100%> (ø)	⬆️
web-extension/popup.js	100% <100%> (ø)	⬆️

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[Pharb]

I added the remaining tests, this PR should be complete now IMHO.

@martinhoefling When your review is done I think we can merge this to master.

[martinhoefling]

Nice, LGTM!