Support tab completing users and joined rooms

**Is your feature request related to a problem? Please describe.**

There is no way right now to directly *mention* another user in a room or one of the joined rooms from the input field other than manually copying their displayname via native context menu which is inefficient.

**Describe the solution you'd like**

Allow tab completing usernames and joined rooms from the roomlist.

Support tab completing Spaces when Hydrogen is space-aware.

**Additional context**

Similar to Element web which supports both or Fractal which supports tab completing usernames.