jQuery (and UI and Migrate) are now loaded separately from the rest of
the JavaScript. This adds at least one HTTP request more but has some
advantages:

* browsers can cache it independently
* the cache is only invalidated when versions update
* we do not apply any transformations (replacements, minimizing, etc) on
  this code anymore which makes our dispatcher faster for the other JS
* browsers seem to load (not execut) both (jquery and other) parallel,
  which might increase download speed a bit

This split allowed for the introduction of a new config: jquerycdn. When
enabled the 3 jquery files are loaded from jQueries CDN. This adds
another two HTTP requests but:

* since it's another host those files do not apply to the 4 request per
  host limit and can be loaded (not executed) in paralell which might
  increase download speeds a bit
* the CDN is distributed worldwide which means files are requested from
  the closest location, increasing the download speeds
* since these files/CDN are very popular, chances are high that people
  already have them cached in their browsers, reducing the download time
  to 0 and effectiely halving the javascript needed to download

The option currently defaults to 'off', but I would argue 'on' would be
the better default.