WIP feat(oauth): Add WebChannel support

@shane-tomlinson @ckarlof feedback?

- [ ] ~~Need to check error reporting~~ https://github.com/mozilla/fxa-content-server/issues/1513 
- [x] Need to fix localstorage WebChannel saving
- [x] Tests
@vladikoff - Do you need to show the ready screen or, can you just complete the oauth flow from "confirm" like the iframe flow? If the receiver of the WebChannel message closes FxA immediately, displaying the ready screen seems superfluous.

https://github.com/mozilla/fxa-content-server/pull/1453/files#diff-1e7023ec1cef9d94ba560d6b63545743R76