MDL-33365 core: Move JS output to body
It turns out that, after all these years, we've been doing it wrong.
The YUI Loader should be loaded at the top of the body, not in the head.
Having it in the head means that the body has not yet loaded, and we see
a number of minor issues for scripts which happen very early in the load
process. One of these is the creation of invalid HTML from the YUI loader
because it is not able to insert in the body as it has not yet been
created.
To be safe, we must move all JS to the same point (because of unknown
dependencies between them). These should be placed as early in the body as
possible.
We also change the way in which the legacy YUI css modules are loaded.
