This is a squash of 38 commits ending with
5b080f979d692804452400ac5bed9b17c50b001e

There's a move afoot to put assets inside individual Django apps,
instead of in upper-level "templates" directories.  These i18n
configuration files have to take this into account.

.underscore files are easy to find by their unique file extension, so
the rule can be simple.  Mako files are .html, so we have to be more
nuanced about it.  We don't want to get pure HTML files, and we don't
want to get .html files for tests, even if they are Mako templates.

Change strings from mako.po into underscore.po, which is finally merged into djangojs.po.

The email texts in lms/templates/emails are not extracted by rake i18n:generate because the babel_mako.cfg does not include them.
And a several emails are not translatable because they do not use ugettext.

We use pybabel to extract strings from third-party apps, put them on
Transifex, then merge them into django.po.

[LMS-1681]