Mark usernames in the forum

Prepend an @ to a username in a post and a link to the users profile page will be generated, e.g. @modulargrid

Stricter validation for usernames

We had a very loose validation schema for usernames. For security reasons from no on only alphanumeric characters and numbers are allowed.
No spaces, Umlauts, Parenthesis etc.
Existing usernames stay the way they are until those users change anything in their profile (email, password). Then they have to adapt their username to the new rules as well.