Implement Funkwhale's new authentication system

[apognu]

Funkwhale is implementing a new authentication system based on OAuth2. I'm not sure when the current mechanism will be retired, but all in all, it will be a breaking change.

I need to start investigating how to implement this, and try and find a way not to break current sessions (that should be a challenge) for people not to lose their settings and downloaded tracks.

[apognu]

Authentication seems to work, although implementation by upstream does not follow the standard, so this requires me to stray a bit from the library I use. This is tracked by the dev/oauth branch.

I still need to handle edge cases, to plan for the migration from the old authentication system to this, to account for possible modification of authorization scopes when Funkwhale introduces new features, and handle errors properly.