Add moderation support

[ghostwriternr]

Moderation in this context means taking actions on the group as an admin. There are a lot of use cases for automated moderation, considering the volume of posts. This issue can be considered closed if R2-D2 can support a simple use case: When a new song is posted, see if the song was shared in the last 90 days and delete it. To test it, initially, R2-D2 can post a comment on a repeated post instead.