MDL-38771 Javascript: Use the YUI combo loader in production

We should always use the combo loader, even when we do not combine the
sources. We add various cache headers using the combo loader and have
methods of invalidating the caches when we need to patch upstream YUI.

This patch keeps the _ability_ to split YUI modules into separate files but
as a developer-only setting renamed to 'yuisplitmodulefiles'.