Implemented CheckBanned and Unban (Attempt two)

Built two additional commands (unban, checkbanned) 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*
checkbanned command: */checkbanned* or */checkbanned user* (shows all banned users or
if a specific users banned status)
I built this PR based on #1075 and it should be noted that the existing
ban command already works on offline users.

I've also added unit tests for both unban and checkbanned commands.

This is my second attempt at posting this. The first time went horrible because I can't GIT. This time I've used an up-to-date branch so my PR shouldn't be stale. Also all commits are squashed instead of a ton pushing.

Hope this is up to your standards, if I can do anything better regarding code style, git pushing, etc I would greatly appreciate the advice!