Synapse should ideally prevent users from creating an adminless room

This issue has been migrated from [#3326](https://github.com/matrix-org/synapse/issues/3326).

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### Description

Currently, it's possible to change the room power_levels such that no users have administrator permissions, effectively making the room uncontrollable by anyone. This seems like a bit of a footgun - i.e. it's possible to accidentally demote or mute yourself, as a sole admin, and be therefore unable to undo the action. Since it doesn't really make sense to have an admin-less room, synapse should ideally enforce that at least one user has the power to send a m.room.power_levels event in the room when a new m.room.power_levels event is submitted, and deny the request if that is not the case.

### Steps to reproduce

- Make a new room.
- Accidentally press the 'mute' button in riot/web
- Oh dear.