Supporter: js: fade out “Expand All” anchor when all sections expand

The expandable sections can be expanded either one-by-one, or with
the “Expand All” button. Add a counter for each expandable
section (which requires their div's to have 'id' attributes, lest
they be counted in the '__global' section of expandables).

The __global counter will work as advertised if you have no 'id'
attributes on any of your 'expandable-section'-classed div's, but if
you mix a __global without an id with ones that *do* have an id, it's
likely this particular code won't work for that.

Finally, add some documentation which is probably over-documenting
for someone who knows Javascript and jQuery well, but it took me a
while to figure out this code so I felt throwing some notes in there
might be helpful.