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.
Footnotes and such on required fields were picking up the red (e.g. the allowed HTML instructions above the banner content field). We only want the red on text in the dt.