Add age option for Abandoned Rooms cleanup function based of last room event activity time

[MurzNN]

Abandoned Rooms function must cleanup all rooms without members. But users can leave and rejoin old rooms later. So will be have an option to keep abandoned rooms some time, for example delete Abandoned Rooms only after 1 month of inactivity (no new events in room).

[cjdelisle]

If the room is invite-only then it is definitely impossible to rejoin so those can be cleaned immediately (if there are people in the room from other homeservers and they re-invite you, your HS can just request the history from theirs).
If the room has no aliases and it is forgotten from your "historical" section, then it is also impossible to rejoin.

In the event that it (remains in someone's "historical" section OR has an alias) AND it is not invite-only, it might be logical to keep it for a period of time, otherwise it can be dropped instantly.

If you could make a pull request to allow this as an option, that would be great.