Implemented modifying peer asignment data in course administration interface

Description and minimun review number can now be modified.

New peer asignments cannot yes be created and EvaluationCriterion are not
yet handled.
Implemented peer review assignments deletion
Change id handling before creation in PeerReviewAssignment
Implemented creating peer review assignments
Define EvaluationCriterionList.url as a function
Fetching peer review evaluation criterions
Allow modification of evaluation criterion title and description
Implement deleting and creating evaluation criterions
Delete submissions and reviews when an assignment is deleted
Modified criterion list loading

Modified criterion list loading to use data as a fetch parameter instead of
ussing the assignment id in the url method.
Fix style issues
Explicit initialization of _criterionList

_criterionList is explicitly set to an empty list when creating a peer review
assignment in order to prevent an error taking place when a peer review is
created after being deleted.
Activate peer review assignment tab after creating a new criterion
Ask for confirmation before removing a peer review assignment
Merge branch 'feature-peerreview' into feature-peerreview-teacheradmin

Conflicts:
	moocng/static/js/models.js
	moocng/static/js/teacheradmin/units-views.js
Merge branch 'feature-peerreview' into feature-peerreview-teacheradmin
Display if a unit has a peer review assignment

Show a "PR" badge in the unit list of the course administration interface in
units that have a peer review assignment
Adding Spanish translations to peer review teacher administration
Use super instead of ModelResource to call obj_delete method of parent class
Fix style issues