1. Add underscore.po into the merge section in config.yaml
2. Using one babel command template to construct two babel commands

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

Currently the strings in .underscore files are not extracted
by i18n/extract.py, which causes most strings in such files
are not translatable. As there is no existing tools which can
extract messages from these files, a simple shell command is
written to do such job.

We already have makedirs_p()

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

[LMS-1681]

Now normal i18n:generate doesn't care about missing files, and
i18n:generate_strict does.  The robot task calls the strict version.

There may well be a better way to make the two tasks, but my Ruby+Rake
skillz are weak, and it's all going away soon.

Add section headings to test console output

i18n tests generate XUnit reports

Move i18n and docs test commands out of tests.rake

Removed unused make_dummy.py
Made dummy.py executable

LMS-1967

We were writing the main file twice.  No need.

When this was conditional, we'd have plural info in some files, not in
others. Then we'd merge them, and msgcat would add wacky markers to
indicate the difference, and the metadata wouldn't parse.

Use the configuration more, and we don't need to delete files we're
about to create.