Token/Invite based registration

[NULLx76]

Registration should happen using discord-style invite links or codes. That is to say per token:

• limited or unlimited amount of uses
• possibly a time limit

Every existing user should be able to generate a token.

A nice to have would be showing who invited whom.

[Druue]

Could maybe add this to some user overview view as well, tagging who invited whom there + invite code if still active?