Support bridging +M channel mode (registered users are unmuted by default)

[poVoq]

Many IRC networks support the +M channel mode, which is similar to +m but automatically unmutes users that are registered through SASL or Nickserv.

This is a very common method to reduce spam while keeping the need to manually voice people in +m rooms minimal.

However when turning on +M in IRC channels with a Matrix portal the very unfortunate situation arises that people on the Matrix are not muted at all and can continue to spam the channel and talk with other Matrix users, but none of that is visible on the IRC side as the Matrix puppets are muted there.

I guess this issue is somewhat related to #553

Edit: I see here that apparently this is called +R on Libera.chat. I guess that might be bridged then?