Implemented IsBanned and Unban

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.