Will/fix i18n in ai branch

My last merge to the AI grading branch broke the i18n scripts because `BASE_DIR` was moved from `settings/base.py` to `settings/test.py`.  This fixes the issue by moving `BASE_DIR` back.

I've also generated translation strings for AI grading, which we'd need to do eventually anyway.

@stephensanchez 
Why does dirname get called twice?