Fixes redirection error for Carrington Blog on Multisite

Carrington was sending admin form requests to options.php. On multisite,
this technique only works if you are fully utilizing the Settings API
(and not even then, if I remember correctly). Since Carrington provides
its own form submission handler, we can use pretty much any other
endpoint for processing form submits. So I've opted for the theme
options page itself.

See #383