Changes to switch to the Rails asset pipeline (Sprockets)

I considered squashing down these commits into one, but I think
the history is more helpful recording each to the changes that
we had to make as part of the migration.

These changes work fine with an empty THEME_URLS; other
themes need to be updated.  I'll shortly make pull requests in
alavetelitheme and whatdotheyknow-theme to switch them.

I added some basic documentation on updating other themes,
and it would be a quick job to do this for other installations if
they need help.  (e.g. doing chiediamo didn't take long.)