Remove mod icons - 2.4's are mostly bigger, cleaner versions

Since 2.4's mod icons are essentially bigger, cleaner versions of the
same icons we're using (for the most part), let's just switch back.
Add new "Use mod chooser tiles" setting

This new setting, when enabled, transforms the list of mods
in the mod chooser into a grid of "tiles" - similar to icons on
most OSes' desktops.  The functionality is identical.
Add headings to and reorder settings page

Settings page was getting to be a mess, due to options being added
without logical groupings or headings to separate them.
Add 'Populate "Courses" menu' setting

This new setting can be disabled in order to prevent the Awesomebar's
"Courses" menu from being expanded to include all categories and courses
(up to the defined limit).
Deal with hidden sections for students properly

As a student, hidden sections threw off the activities in subsequent sections.
Make AJAX "Move" icon look better in Edit button menus

Put it neatly in the list with a caption beside it (albeit after 1.5s).
Version bump to 1.8