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
@zaach was giving me the hard sell over lunch about this. His argument is that it would be a PITA to use `Assertion` if you have to pass it, e.g., a half dozen other objects just to use it. 

There's some meat in that argument. It may depend on how many places you create `Assertion` objects and how many dependency objects you need to give it. 

I'm happy to keep it how you have @shane-tomlinson, but this is one issue to keep an eye on.