[FR] Add ActivityPub support.

[RokeJulianLockhart]

Please describe the problem / need you are trying to solve.
I want to be able to subscribe to changes and publish them (publicly and/or privately; it's server-dependent).

Describe the feature or the improvement you'd like and what are you trying to achieve.
Publish activity via ActivityPub support to allow this.

Describe alternatives you've considered
RSS, although this doesn't provide any interactive functionality.

Additional context
https://fediverse.party/