Rename an accordion in theme editor to avoid conflict with '#navbar'
Split some Bootstrap variables into variables_custom.less

The theme editor is set up to adjust several key variables used
by Bootstrap, as well as any additional variables that are defined
by the current theme.  However, there was no way for the key
(global) variables to be identified until they had been set.

This change moves those variable definitions into a separate file,
variables_custom.less, and adds that file to the search path for
ThemeController.find_less_variables().  That way the ThemeController
always knows about them, and you can view/modify their values in
the theme editor like you're supposed to.
Add default palette options to theme editor

The palette feature in the theme editor is a bit frustrating when
there aren't any colors to start with; you'd have to add all of them
yourself.  This change looks at all of the current LESS variable
values and makes sure that any colors are included in the palette.