Change how jquery.js and jquery.dataTables.js dependencies are handled.

In response to #1992.

1) Includes the original source, rather than minified source.  (Since these files are used locally, I see little advantage to including minified source at all).

2) Moves them to `astropy.extern`.

3) Adds their licences to the licenses directory

4) Adds configuration options to select alternate URLs to find these files.  These, presumably, will be patched by Debian so the default refers to the package-installed versions of jQuery and jquery.dataTables.