Allow deleting rooms with local users

**Is your feature request related to a problem? Please describe.**

It's a long known bug that Synapse corrupts room data from time to time. There needs to be a way to delete these rooms – it's not possible to leave them via clients (results in Synapse exceptions).

**Describe the solution you'd like**

If there are local users in a room, the new delete room should have a warning like "There are x local users in this rooms" and a checkmark "force delete". If checked, it would delete all memberships as well.