Create mod actions database table

[Zabuzard]

There are quite some features being implemented right now which are blocked by having a way to retrieve moderative actions that have been executed in the past:

• Warn command Migrate ban/warn system #63 (to have a place to store the reason)
• Audit command Migrate /warn command #249 (to get a list of all actions for a user)
• Temporary bans Migrate temporary ban/mute #267 (to undo the action)
• Mute command Migrate mute, unmute commands #252 (to reapply the role after leaving)
• Actual author in log messages instead of bot Mod audit doens't log who issued moderation command #280 (to figure out the actual author behind the command)