feat: Workspace join improvements

[jo-gross]

The current way to join an existing workspace is to copy the workspace id (indeed there is an button for that), but nobody must accept this.
Therefor the join system should be reworked, by introducing a random (shorter) code to join a workspace. And then an admin or owner must accept this invitation. Maybe this is the start for an email notification service.

• Random join codes
• Before letting someone in, make an "WantToJoin"-list, where admins cann reject or approve
• Email notifications (if someone wants to join, if your request was accepted/rejected)