[DEPRECATE] Matching exercises are now card sort exercises

[Rob Beezer]

"Matching" problems began being a 1-1 correspondence. That was relaxed to allow
for multiple "premise" (the draggable) to be associated with a single "response"
(the drop zone), and perhaps this is when more distractors became possible. The
markup was designed in part to reflect the interface, "drag-n-drop". Cart
before the horse, and my mistake.

This type of problem might be more accurately called a "card sort". So we have
made a cosmetic change: the "matches" element has become a "cardsort" element.
So you will want to make that change in your source. Until then you will get
warnings, but your output should be unchanged until you get around to it.

Why? There have been requests for exercises where a matching is wide-open. For
the mathematicians, less like a function and more like a relation. We are going
to add *new* markup for this even-more-relaxed version, since it won't be as
convenient to write as a card sort problem. Thus, the two very different styles
of markup will both be useful.

Look for new markup announced here after Brad and I do some coding and testing.
Also: the static version of the existing card sort version has not kept up with
changes from the initial relaxation. That's on hold until we have both types of
problems.

Thanks to Brad for his collaboration on this.

Rob