Add some brokers

This is for @nchapman, @zaach, @vladikoff and @ckarlof. These are the first brokers, see what you think!

feat(client): Add some brokers!

* Start to add brokers that know how to startup/complete flows.
* 3 brokers to start, fx-desktop, redirect, web-channel.
* Get rid of the Channels high level abstraction, these are used directly by the brokers if needed.
* Move some code from service-mixin and various channels to brokers.
* Update tests to make use of the brokers.
The assertion library is causing a similar circular dependency for me while trying to add the `User` model. User -> ProfileClient -> Assertion -> FxaClient -> User. It seems like we can trim FxaClient's concerns further by moving account storage to a different layer (maybe in the views directly) and pass credentials explicitly to each method.