Use `static` templatetag rather than `STATIC_URL`

This allows us to use the CachedStaticFilesStorage which gives unique
names for each version of a static file.
Merge branch 'next-staticfiles-support' of git://github.com/evansd/feincms into next

* 'next-staticfiles-support' of git://github.com/evansd/feincms:
  Use `static` templatetag rather than `STATIC_URL`