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.
I am mulling this over, I can see pros of forceAuth living in either place. The only way `isForceAuth` is `true` is if the RP sends the user to `/force_auth`. Because the intent was signaled by the RP, I thought it made sense to put it on the Relier.