Update with changes suggested by @nchapman and @zaach.

* Clean up Relier creation.
* Remove the "isSync" from the context of views that do not need it.
* Add a `Base` Relier that can be used as the base for other reliers or as a Null relier.
* Always pass in a translator into the FxDesktopRelier.
* Always pass in an OAuthClient to the OAuthRelier.
* Always fetch the webChannelId from the relier instead of ad-hoc.
* in fxa-client, ditch the relierHas and relierGet functions.
* Correctly set the default values in the models.
* Fix OAuth users who verify in a second browser not being shown the redirect link.
* Fix Sync users who verify in a second browser.
* Add unit/functional tests for areas we were previously missing tests.