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.
Similar to [above](https://github.com/mozilla/fxa-content-server/pull/1609/files#r18864578), I don't see the `webChannelId` as a property of Relier, but instead of a property of the communication /protocol.