jQuery: update vendored version (1.9.1 -> 1.12.4)

The vendored jquery version was 1.9.1 from 2013-02-04. Let's replace it with the
most recent one from the 1.x branch (1.12.4 from 2016-05-20).

The modification in rjquery.js is needed because recent jQuery versions changed
their behaviour, and do not set themselves on the global window object.
See: parcel-bundler/parcel#333 (comment)

This will be the lastest jQuery 1.x version ever, because 1.x branch is
definitively EOLed (see jquery/jquery.com#162).

This is a stopgap measure to get the latest security fixes. Going forward,
another strategy will be needed.

Closes #3640