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
>>  Do you need to show the ready screen

> Yeah I think the UX team prefer it that way. It makes it less surprising to the user.

I'm trying to understand the WebChannel flow and we may be thinking of different cases. There are two cases in which ready.js is rendered for an email verification, the first is in the original tab/dropdown once the poll in confirm.js has completed, and the second is in the window the user does the verification in.

When the user completes their email verification in the WebChannel flow, is the original window with the "confirm your email" screen still open, or has it been shut?

What is the effect of the call to `submit`? A message is shipped across the WebChannel of oauth_complete, what does the browser do at that point?