Add ActivityPub core functionality

[Floppy]

Prerequisites:

• ActivityPub architectural design #1389
• Proper multi-user capability #647
• Add social features (feeds & following) #1690

NLNet milestone 3.4

[Floppy]

https://gitlab.com/experimentslabs/federails will be our friend here. It's taking the same approach I was intending to, of building a generic Rails engine that you plug into your existing system to layer on activitypub support.

[Floppy]

http://pubkit.net/ will be VERY useful for debugging.

[Floppy]

The new social core of Manyfold is built around ActivityPub, so we kind of did this the other way round from originally intended!