Remove styles for toggling the builder section list

Remove vestigal styles for controls that were used to show and hide
the section list (we've also called this the table of contents).

The section list has been always-visible for a very long time in the
app and there aren't any plans to make the section list hidable, so
remove the clutter of these unneeded styles.
Hide add new section icons when reordering sections

Use CSS to hide the spacers and add new section icons in the section
list when a section icon is being dragged to reorder the sections.

An extra "+" icon is still visible when the section icon is
"dropped", before the Backbone collection has finished syncing with
the server, triggering a re-render.  This is because the the CSS class
added by jQuery UI is removed before the Backbone collection syncs.
Fixing this is a more complicated fix that probably involves
optimistically updating the UI before the server syncs (and providing
a mechanism to undo).

Closes #905