WP 4.1 - Trying to upload new header image shows a persistent overlay

As described in [this support thread](http://commonsinabox.org/groups/help-support/forum/topic/functions-php/), there are a number of problems with the CBOX Theme Admin UI. The two commits in this PR paper over two of these issues.

The `_muDialog.dialog('destroy').remove();` error remains unsolved.