Permanent mute

• Requires permissions as a base, ref https://github.com/nextcloud/spreed/issues/931

Is your feature request related to a problem? Please describe.
In a conversation with many participants it might be necessary to mute people when too much talking is happening in the background and disturbing a presentation. There's already a force mute feature, but after muting, people can still unmute themselves again.
A moderator might want to permanently lock the mute state for all participants until it's their turn to speak.

Describe the solution you'd like

• A new call option "Participants cannot unmute themselves, only moderators can" that could be used for such meetings.

  • The option would be visible in the three dots menu and specific for the current call and can be changed during the call, to switch modes.
  • Optionally, that new option could have a default for a given conversation (like a classroom)

• A moderator can manually "ask to unmute" anyone in this mode to give them a voice.

• If someone wants to speak, they can raise a hand

Review the existing force-mute behavior and the permissions requirement in issue #931, then compare the related raise-hand request in issue #4562. Done means a per-call moderator-controlled option prevents participants from unmuting, supports moderator requests to unmute, and is available from the three-dots menu; a conversation default is optional.