Room write locking

### Your use case

#### What would you like to do?

It would be nice to have some write locking mechanism for defined rooms. Something like a additional dialog "really write?"

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

You may have the situation too that you sometime wrote posts into the wrong room. This is unfavourable for large rooms with spamming rules, for example. For these rooms a write locker would be very useful.

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

Additional parameter like a favorite flag to opt-in locking, dialog when sending a post

### Have you considered any alternatives?

We already have message deleting of course but this does not work e.g. on bridged irc channels.

### Additional context

I am not sure if this needs a spec addition. Favorites do have a parameter in the protocol? So probably not trivial to implement but really helpful.

Start with the issue's proposed room-level opt-in lock and confirmation dialog, then determine whether the behavior belongs in an Element client or requires a protocol/specification change. Check how room favorites and message sending are represented before defining the affected entry points. Done means the scope, client coverage, configuration source, and confirmation behavior are agreed and documented.

Written by the indexing model from the issue text.