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
Trim teacher input when adding peer review exercises
Strip whitespaces in teacher input in the API
Rotate line chart labels to avoid text cutting

And update the nv library too.
Fix option's urls. Adding slash at the end.
Cache the js catalog url
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