Jarv/config refactor wip

This wouldn't do anything, no? `ADDL_INSTALLED_APPS` would be `[]` from `common.py`? You might have meant to do the same thing as in `lms.py`: https://github.com/edx/edx-platform/pull/6096/files#diff-cf54381decd24863407f3f28b546a972R210