Don't delete students' submissions and reviews...

...when a peer review exercise is deleted. The data in the mongodb is not
removed in cascade when there are changes in the postgresql.
Adding mathjax feature
Update idp.py

Fix enrollment bug.
Merge pull request #28 from kaleidos/feature-mathjax

Adding mathjax feature
Minor improvements when invoking mathjax
Merge remote-tracking branch 'upstream/master' into feature-openbadges

Conflicts:
	moocng/settings.py