Extract i18n strings from *.underscore files using django-babel-underscore

After #3704 was merged, I looked around to see if there was a better way to accomplish the same thing, and I discovered [django-babel-underscore](https://github.com/EnTeQuAk/django-babel-underscore), which looks like exactly what we want. I removed @louyihua's code and configured our babel extractor to use this plugin instead. @louyihua, can you verify that this still extracts all the strings that need to be translated? @nedbat, can you review this?

Also adding the devops team since this adds a new dependency in our codebase.