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.
Another possible change is to rename this function to something like `prepareLinks`. This could be called from the generic `signin` and `signup` views, eliminating the need for the `oauth_*` overrides. The default `prepareLinks` would leave the links alone, the OAuth version would munge the links to the oauth equivalent.