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.
How do you think we should handle this? Some ideas:

* Set a flag in sessionStorage whenever the user stages an email for verification, and check that in service-mixin before finishing the flow. 
* Notify the brokers when an email is staged, check the broker in service-mixin->finishOAuthFlow before actually finishing the OAuth flow.
* Need more ideas.