feat(oauth): Add WebChannel support

@shane-tomlinson @ckarlof feedback?

- [x] ~~Need to check error reporting~~ https://github.com/mozilla/fxa-content-server/issues/1513 
- [x] Need to fix localstorage WebChannel saving
- [x] Tests
- [x] Add decorate
- [x] Fix timeouts for signin / signup on errors