Replace require_scope! and other OAuth helpers

[toastercup]

There are now Doorkeeper-specific libraries that provide DSL extensions for Grape for enforcing scope requirements, as well as various helpers for grabbing access token, etc. We should switch to using them, especially as Doorkeeper has changed their API a decent bit.

See:
https://github.com/fuCtor/grape-doorkeeper
https://github.com/antek-drzewiecki/wine_bouncer
And in Doorkeeper 3.x: https://github.com/doorkeeper-gem/doorkeeper/tree/master/lib/doorkeeper/grape

[toastercup]

Fixed by #238