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.
Added unit tests for unban and isbanned

Simple unit tests for Unban to handle
- Missing username
- Non-Admin use of command
- Unban nonexistent user
- Cannot Unban another Admin
- Cannot Unban yourself
- Unbanning a banned user

for IsBanned to handle
- Get banned user list
- Non-admin user
- Non-existent user

Updated test when trying to ban yourself to appropriate exception text

Updated LanguageResources message for Unban_UserRequired
Name change: isbanned > checkbanned

All instances of isbanned are now checkbanned for commands, with
appropriate casing for classes.
Merge pull request #1083 from JonFriesen/dev

Implemented CheckBanned and Unban (Attempt two)