3870 Tidy up admin settings page

https://code.google.com/p/otwarchive/issues/detail?id=3870

* Put checkboxes before the labels so the labels aren't so squished
* Break it down into field sets so similar settings are grouped together
* Change the admin banner section to a text area so you can read the text

When I went to check my HTML, I found a problem in the header, so that's also fixed here.