Add option to preserve room history on room deletion

[Fishbowler]

When a room is deleted, the history isn't visible in the places we expose it (e.g. the public archiving on https://hostname:7443/monitoring/).

Add an option to retain the ability to view history for deleted rooms.

[guusdk]

Note that when retaining the room history, it must be ensured that this room history is not associated with a newly created room of the same name.

This could relate to tombstoning: https://igniterealtime.atlassian.net/browse/OF-2760