Change videos by thumbnails
Merge branch 'master' into improve_unit_list
Fix type, fix styles to handle thumbnail image
Open a modal window with youtube player on thumbnail click
Check if the property exists before striping it
Ensure the properties are not None before stripping them
Correct deleting peer review criterions

Explicitly conterting to int id of criterions in iterator function
Implemented a simple testmail command
Change subject to every mail to was sent
Merge master in improve_unit_list branch
Stop video when user close the media modal
Merge branch 'improve_unit_list', remote branch 'origin'

Conflicts:
	moocng/static/css/moocng.css
Handle knowledge quantum being deleted on peer review assignment cache invalidation
Compress and unify the CSS files
Make the MongoDB connection reused among the server workers
Better usage of django compressor

Prepare it for using offline mode in production
Remove unused variables
Fix bug related to the teacher invitation when teacher not exists in the moocng and log with SAML
Replace django_gravatar with a custom mini implementation that is the same but mark the strings with mark_safe so the gravatar URLs are not escaped
Don't invalidate teachers list chache when deleting a course