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.