Currently no row spacing exists in the command completer. Add row spacing
to make it prettier.

This is still a bit flaky around when to remove a mention, but it should
work in most cases. Might add a toggle in the future to disable these
though.

See also:
https://daniel.haxx.se/blog/2023/01/08/copyright-without-years/
https://hynek.me/til/copyright-years/

fixes #429

- Add scrolling
- Only display ~7 items at once

Fixes #578

When no image is set for a room, the room didn't have the first
character in the avatar, when opening any completer, that showed it.