Migrate FxA implemetation to WebChannels instead of interceptor

[keianhzo]

Is your feature request related to a problem? Please describe.
Since we're using Gecko, we should use WebChannels for communicating with the FxA web content, instead of intercepting URL loads.

Based on this comment: #1973 (comment)

Additional context
Once we integrate the webchannels support, for free we get handling of "Choose what to sync" selection during FxA sign-up, which our current interceptor-based integration doesn't support. Also with a tiny bit of extra work, ability to add "Manage Account" button to your FxA settings page.

This requires WebChannels webext integration.

[keianhzo]

@jvonitter We better push this to v9. We have decided to talk to the AC team in the All-Hands and expose them the need of making AC more aware of FxR in general. This is part of that conversation. We should provide time estimation and a proposal to evaluate and maybe contribute.

[keianhzo]

This is not going to happen for v9. We decided to do a staged migration to better integrate AC-FxR. First stage completed, second should add support for WebChannels in FxR without needing to implement the whole Engine/EngineSession interface.

[bluemarvin]

This is still blocked on a-c

[keianhzo]

@emstanley After the PR lands we are good to start working on Addons support and FxA "Manage Account" settings.