jquery.tablesorter: Move files to their own directory

jquery.tablesorter: Move files to their own directory

* Reduce clutter in src/jquery/.
* Make it easier to see which files belong to which module, especially
  src/jquery/images/ which was confusingly shared between multiple modules.
  This also helps emphasize that while jquery.tablesorter.less is included,
  jquery.tablesorter.styles.less is not.

Also follows-up 8cdfcc5fd, which placed its style-module definition among
the 'mediawiki.*' modules instead of near the 'jquery.*' modules.

Change-Id: Ib639de5df323a36982ecdd89158a939beaaa2dd3