* Remove AbstractOAuth2Controller
There is no reason to provide an implementation for a specific oAuth2
library in core; it's not generic enough (eg. auth-twitter can't use it).
This code could be moved into another package which auth extensions
depend on, but it's a negligible amount of relatively simple code that
I don't think it's worth the trouble.
* Introduce login providers
Users can have many login providers (a combination of a provider name
and an identifier for that user, eg. their Facebook ID).
After retrieving user data from a provider (eg. Facebook), you pass the
login provider details into the Auth\ResponseFactory. If an associated
user is found, a response that logs them in will be returned. If not, a
registration token will be created so the user can proceed to sign up.
Once the token is fulfilled, the login provider will be associated with
the user.
They will probably be refactored away at a later stage (when we get
rid of the command bus). Until then, this lets us remove the
Flarum\Core namespace and actually feels quite clean.
