Prevent user from self-demoting to PL-1 (self-mute)

### Your use case

#### What would you like to do?

Prevent self-demote to -1.

#### Why would you like to do it?

There is no value in a user self-demoting to -1. They can't get out of this mode and once there they can't use widgets, which breaks some workflows.

#### How would you like to achieve it?

Disallow in the UI.

### Have you considered any alternatives?

* Build a synapse module which prevents the PL change
* Server capabilities define the features on the client (this is a long term hope)

### Additional context

_No response_