Expanding ban command feature set (unban and isbanned)

Built two additional commands (unban, isbanned) and made some changes to the existing ban command.

**Changes**
- ban now shows notification to admin user in bad
- custom message if admin tries to ban themselves

**Additions**
- unban command: /unban username
- isbanned command: /isbanned or /isbanned user (shows all banned users or if a specific user is banned)

I built this PR based on #1075 and it should be noted that the existing ban command already works on offline users.