Merge pull request #3971 from louyihua/django-babel-underscore

Django babel underscore
extract from underscore files under cms, lms, and common

to avoid extracting from test_root and such
Don't search for underscore files under lms
Babel underscore extraction update according to @nedbat

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