4075 New admin banner system

https://code.google.com/p/otwarchive/issues/detail?id=4075

Moves the banner out of the settings table and into its own table and gives the banners a makeover, allowing us to have different styles for alerts, events, and general notices.
Multiple banners can theoretically be set to active at the same time (although, in reality, they are unlikely to be, since our banners are usually time-sensitive and we turn them off when the information ceases to be relevant). We only want to cache and use the most recently created one.