Rooms should be unpublished automatically when the last member leaves

*This issue was originally created by [**@neilalexander**](https://github.com/neilalexander) at .*

Rooms published to the public room directory should be automatically unpublished when the last member leaves the room, since the room will become un-joinable at that point.

Start by tracing the room-membership leave handling and the public room-directory publication logic in Dendrite. Verify the behavior for a published room when its last member leaves; done means that room is no longer published and remains un-joinable as described.

Written by the indexing model from the issue text.