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: https://github.com/parcel-bundler/parcel/issues/333#issuecomment-357882648

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

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

Closes #3640

src/static/js/rjquery.js

@@ -1,5 +1,5 @@
 
 // Proviedes a require'able version of jQuery without leaking $ and jQuery;
-require('./jquery');
+window.$ = require('./jquery');
 var jq = window.$.noConflict(true);
 exports.jQuery = exports.$ = jq;