refactor(client): Create one FxaClient and one Relier in app start.

* Remove the singleton FxaClient created in views/base.js, instead create one in app-start and pass it via the router to all objects that need it.
* Update assertion.js to use a passed in fxaClient instead of creating its own.
* Update the tests that need to be updated.

Part of issue #1623
@ckarlof - busted. Officially, yes it does. FxaClient does most of its relier access via helper functions that check for the existence of this._relier. I did this to avoid creating FxaClients with Reliers in every test. Lazy. Will update.