We recently moved to a new Django Pipeline and Babel compiler for our
static media, but were hitting caching problems when moving between
branches, with other files being served up.

While investigating the caching issue, we found that we were still
loading static media from the app directories, and not from the
directory containing the generated output (htdocs/static/). This meant
the wrong files and the wrong mtimes were being considered.

It's unclear why this was a new problem. The cause of the caching issue
was that mtimes were being compared, and Git preserves mtimes for files
when switching branches, even if the content changes. The server was
sending Last-Modified headers containing these mtimes, and the browser
was sending If-Modified-Since with the same mtimes. So the caching issue
should have happened before, and perhaps it did but we didn't notice in
practice (until now).

Either way, switching the location fixes the caching issues, and is more
correct.

Cleared browser cache and removed htdocs/static/.
Accessed /js-tests/ under release-2.5.x.
Switched to the branch for this change (under release-2.6.x).
Reloaded the page
Saw that the new media was loading, rather than serving up the stale

   content (as before).

I also verified through temporary logging statements in Django that we

were now accessing from the correct path.