Manually approve follow requests

[Floppy]

Following up #2774, users should be able to accept follow requests manually. Models, creators, and collections will still be automatic.

[Floppy]

Users don't have the Followable concern at the moment, so don't have the callback set up, so actually there might not be anything to do here. It depends on whether following users is something we actually want people to be able to do. And I'm not sure it's really necessary yet.

[Floppy]

Users are now followable, so we could do this.