* 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.
* Run extenders exported by extensions
* Add some basic extenders
* Patch Mithril as the very first thing so extension code can run safely
* Load the payload into the app before booting extensions
* Setup default routes before booting extensions
* Replace gulp with webpack and npm scripts for JS compilation
* Set up Travis CI to commit compiled JS
* Restructure `js` directory; only one instance of npm, forum/admin are "submodules"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory
