Always query the static storage for paths, and remove media serials.

We now always go through the static storage module (using the static()
templatetag) to get the path to a media file. This will allow us to get
versioning as part of the filename for free. Since we now have
versioning, we can nuke usage of MEDIA_SERIAL.

JavaScript no longer tries to build up static media paths itself. Now it
uses this STATIC_URLS dictionary that maps known media files to paths.
Later, once we have a sprite image, we should be able to remove even
this.

We do still generate cache serials at startup, though, since we use
AJAX_SERIAL in places. However, we no longer scan the media files at
all, only templates, meaning we get a small performance boost.

Played around with the UI. I didn't see any problems.

Verified that the paths we were generating for JavaScript were correct.

Unit tests pass, but these are mostly template changes, so no big surprise
there.