Add Basic Moderation System

[andrewlalis]

Add all the components needed for server moderation:

• Muting users
• Banning users
  This should work for users that are in the server, or even those who have already left. It should (optionally) remove all messages the user has sent, from all channels.
• Warning users
  We have discussed previously that it would make more sense to use a weighted warn system instead of just "3 warns and you're out". Moderators should, when warning someone, pick a choice for what type of warn it is, and this type should have an associated severity. Once a user has accumulated enough warnings such that the combined severity exceeds some threshold value, they should be banned.
• Removing messages
  Should be similar to the old purge command, although Kemikals has mentioned that it would be convenient to also be able to specify the id of a message whose timestamp marks the boundary for which messages can be deleted.
• Add Spam Filtering #9
• Server raid protection #10
• Phishing link and advertising protection #11