[feature] Add ActivityPub support to federate with the Fediverse

ActivityPub activitypub.rocks enables data entered in one instance of a software, say an OpenKi instance to be consumed by displayed by an instance of another software, be it another OpenKi instance or an instance of say Mobilizon.

Data from the second instance may indeed be consumed and used by the first instance of OpenKi too.

The development of Mobilizon focuses on ActivityPub and has in the development roadmap features which are similar to the features of OpenKi.

Implementing ActivityPub may increase the activity of instance through the visibility of courses organised in one region using OpenKi and events including courses organised in another region using Mobilizon.