Allow Nickname Changes for Banned Users and Apply Name Filter

**Issue:**

If someone is banned for a family-unfriendly nickname, then the moderator team has to permanent ban him, so he approaches the team for a rename. Otherwise, the offender would simply ignore the request for a nick change.

This creates an additional workload for the team.

**Solution 1:**

Allow the banned user to login into the website _only_ be able to rename himself.

**Bonus Solution 2:**

A pre-check for nicknames when registering/changing, so the most common violating names are filtered. If someone is dodging that, then further actions can be taken regarding his intended malicious behavior from the moderator team.