AP: Support collections other than followers as recipients.

[Laurin-W]

Issue
Currently, activities can only be addressed to individual actors or to the followers collection. Other collections, like the apods:contacts are not resolved.

https://github.com/assemblee-virtuelle/semapps/blob/master/src/middleware/packages/activitypub/services/activitypub/subservices/activity.js#L49-L59

Components affected
SemApps ActivityPub package.

Proposal
Support resolving arbitrary collections of actors, as long as they are in the actor's pod.