Changing default power level may mess up room or itself

### Description

Element tries to be too smart about named power levels, and consider "Default" something equal to the **Default role**.
Changing the *default role* however makes truly messed up user experience. I have tried to set it from 7 to 1, and it worked. It became the "new Default".

Later I wanted to set it back to 7. UI don't show a number anymore (in fact shows just the same Default) so I click custom then set 7. Then, since there are no exit, done, save or something I click outside. Element fires a power level. With `default: 1` Going back it shows "Default" as "1".

Tried several times, stays 1 and new power level events are fired with same content.

So tried to set it to 7 _and_ set Send Messages to something else (5), and both were updated properly.

### Version information
- **Platform**: web v1.7.16
- **Browser**: Chromium Version 87.0.4280.88 (Developer Build) built on Debian bullseye/sid, running on Debian bullseye/sid (64-bit)
- **OS**: Debian/sid
- **URL**: https://riot.grin.hu/