Allow heads-up notifications from designated rooms

### Your use case

#### What would you like to do?
I would like to be able to set notifications from specific rooms to display heads-up notifications (dropdown bubbles from the top of the screen in addition to sound/vibration, lock screen, drawer, and status bar)

#### Why would you like to do it?
One-on-one or small group chats are generally important enough that I'd like to be notified immediately of messages without having to open the drawer. This is consistent with the default behaviour of Signal, Whatsapp, and SMS on (probably among others) on every phone I have used. I don't want this behaviour from larger rooms, so I'd like it to be configurable on a per-room basis.

#### How would you like to achieve it?
I would like to create a new notification channel with PRIORITY_HIGH and allow the user to designate certain rooms as high-priority in the per-room notification settings page.

There is a history of similar feature requests for more granular notification control (#2982, #2572). I am happy to contribute a PR for this, but thought I would first open discussion on whether it is wanted at all and if so on the details of what notification levels we should allow and how we should communicate them to the users.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

### Are you willing to provide a PR?

Yes