Load static media from the correct location, to prevent caching issues.
Load static media from the correct location, to prevent caching issues.

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.

Testing Done:
1. Cleared browser cache and removed `htdocs/static/`.
2. Accessed `/js-tests/` under `release-2.5.x`.
3. Switched to the branch for this change (under `release-2.6.x`).
4. Reloaded the page
5. 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.

Reviewed at https://reviews.reviewboard.org/r/8026/

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