WIP: Initial activitypub support

The old MR has rapidly expanded in scope so I'm creating a new one.

This MR adds a generic federator and publisher modules which allow for easy expanding of the federation capabilities to other protocols and a dedicated AP publisher. It also adds necessary functions for receiving and handling incoming activities. On top of that it adds modules necessary for integrating AP functionality with MoodleNet core.

• Pushing activities (almost done, missing resolving follower collections)
• Receiving incoming activities
• Handling incoming activities (basic types done)
• Support for group actors and AP collections
• Handling activities on MoodleNet side
• Adding hooks to push activities to the ActivtyPub module
• API for generating and federating basic activity types
• Syncing remote actors with MN database
• Updating local actors (This will only happen in one case - when the AP module generates an RSA PEM key to sign activities)