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.
There may be some DRY opportunities here. This `_send` looks pretty similar to this one: https://github.com/mozilla/fxa-content-server/pull/1609/files#diff-d548a17ac012756e5e531925eb901932R103