Add missing file: templates/themes/custom_vars.less
Add docstring and clarify code in ThemeController.list_filenames()
Add docstring and clarify code in ThemeController.get_config_form_class()
Use string substitution in Javascript used to compile CSS from LESS
Split up some file manipulations onto multiple lines
Fix string substitution error
Insist that variables.less is included first if it exists

The theme editor compiles a group of LESS sources into one CSS stylesheet.
The LESS files provided by the theme are concatenated together after the
Bootstrap LESS code.  If the theme includes a variables.less file (with
customizable parameters for the stylesheet), then it should be included
first, before any other LESS code that references it.  So, search for
variables.less first, then for any other .less files.