Unread meta space

### Your use case

#### What would you like to do?

As a user I want to have a place where all unread rooms/DMs are listed,
so that I can quickly catch up.

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

Currently it is already possible to change each room list order to show unread rooms at the top. However it is quite a bit of work to click through all spaces and sections (DM, room) until I've found all unread rooms. With an „Unread“ meta space this could speed up things :rocket:

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

Introduce a new „Unread“ meta space. Similar to the „Favourites“ space but displaying all rooms/DMs with unread messages.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_