Fix building localization files on modern versions of Django.
Review Request #10744 — Created Oct. 9, 2019 and submitted
Our build-i18n script, which wraps the building of localization files,
had a failed import on modern versions of Django. We previously were
attempting to import and run the
compilemessagescommand class itself.
Now we're doing it correctly by telling Django to invoke the command,
letting it handle the proper lookup and execution itself.
Successfully built packages against Django 1.11.