Fix building localization files on modern versions of Django.
Fix building localization files on modern versions of Django.

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 `compilemessages` command class itself.
Now we're doing it correctly by telling Django to invoke the command,
letting it handle the proper lookup and execution itself.

Testing Done:
Successfully built packages against Django 1.11.

Reviewed at https://reviews.reviewboard.org/r/10744/

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