Add an endpoint to get the username of another user

[Maxi Herczegh]

Is your feature request related to a problem? Please describe.
We need to get away from UUIDs in the frontend. There is currently no way to display users with user names, we have to display UUIDs and that's not pretty and not at all informative.

Describe the solution you'd like
Add a new endpoint that takes a UUID and responds with the username, maybe the last-seen date and later possibly the online status of the corresponding user.

Additional Context
Please implement this endpoint inside the server.