Add an Admin API to forget all rooms with 0 local members

[Twi1ightSparkle]

Description:

Since forget_rooms_on_leave defaults to false, enabling forgotten_room_retention_period on your server may leave you with a bunch of old un-forgotten rooms on your server. An Admin API to forget all local rooms with 0 local members would solve this and allow you to easily clean up old unused rooms after enabling forget_rooms_on_leave and/or forgotten_room_retention_period.

The alternative would be to manually look over all the rooms on your server and manually deleting all rooms with 0 local members. But this can be tedious if you have lots of rooms or users.

[Twi1ightSparkle]

Also because of matrix-org/matrix-react-sdk#12286, there is currently no way for users to manually forget rooms they have left using Element