settings: Help caching with content-aware static file names

settings: Help caching with content-aware static file names

If we always serve the "style.css" file, even when its content changes,
we run into the situation where the browser (or any HTTP aware middle
man) will cache the file and not download a newer version.

This can be solved by appending the file hash to its filename. If the
content changes, the file name changes and the new version is used right
away. We can also use far future Expires headers with such static files,
they will really never change.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
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v2: Disable for Django < 1.7