Prevent JavaScript errors on the Manual Updates pages.

The Manual Updates page tried to load the JS catalog, needed in order to
expose gettext() for the other scripts being loaded. That path wasn't
explicitly allowed, resulting in HTML being sent back as the script
contents, causing the browser to show some errors.

We may want to consider just blocking off all script loading. That will
require blocking extensions as well, though, and possibly daeling with
other corner cases. In the meantime, allow the JS catalog to be loaded.

Reviewed at https://reviews.reviewboard.org/r/5800/