Reorder cover art: calculate positions differently

On the Reorder Cover Art form, set the values of the hidden input
fields giving the new position in one pass immediately before
submitting.

Previously the values were updated whenever an image was moved, by
swapping the values on the moved image and its neighbour. This was
less clear (in my opinion) and could only cope with simple movements
(one place to the left/right).
Add cover art: determine position differently

On the Add Cover Art page, determine the position where the new images
are to be inserted only when the form is submitted. This has the same
advantages as the corresponding Reorder Cover Art change in the
previous commit.

A small helper function set_position is used because the same code is
needed twice, for the AJAX uploader and the old one.
Simplify Add/Reorder Cover Art code

Get rid of two parameters of reorder_button that (after previous
cleanups) are always null or a no-op function, respectively.
Additionally, merge the image_position function into reorder_position.
MBS-5869: Wrap around when moving cover art
MBS-4374: Reorder cover art via drag'n'drop

Allow cover art to be reordered via drag'n'drop, using the jQuery UI
Sortable component. This also applies to the Add Cover Art page: The
"new image goes here" placeholder can be moved in the same way.
Tweak Add Cover Art image placeholder CSS

The placeholder for the new image on the Add Cover Art page was wider
(and higher) than planned because of its border, displacing it
slightly from the center of its container. Reduce sizes a bit to fix
this.
Minor cleanup to QUnit tests

The diff is huge because of all the reduced indentation. I haven't
changed the behavior of any tests, and they all still pass. My changes
here comprise:

 * Removing verbose references to 'QUnit', and using the global
   functions they provide for us.

 * Removing the 'Run' functions and other unnecessary wrappers. Tests
   are detected and run automatically by QUnit.

 * Moving global test data/helper functions to appropriate module setup
   and teardown callbacks.

 * Moving any DOM modifications required by certain tests to
   #qunit-fixture, so that they're automatically removed for us between
   runs.

 * Adding a small license header atop each file.
Merge remote-tracking branch 'chirlu/mbs-4374' into beta

* chirlu/mbs-4374:
  Tweak Add Cover Art image placeholder CSS
  MBS-4374: Reorder cover art via drag'n'drop
  MBS-5869: Wrap around when moving cover art
  Simplify Add/Reorder Cover Art code
  Add cover art: determine position differently
  Reorder cover art: calculate positions differently
Merge remote-tracking branch 'bitmap/qunit-cleanup' into beta

* bitmap/qunit-cleanup:
  Minor cleanup to QUnit tests