copying out existing versioned-serializers code into shared/serializers/, adding into available serializers, doing proper imports and references.
cleanup from debug session that I missed in initial self-code-review.
Merge branch 'develop' of github.com:learningequality/ka-lite into save_the_django_serializers
cleaning cruft from django's serialize object
re-enabling fixture tests
OMG a merge conflict in a docstring. LOL
Show valid link in alert for tabular coach reports.

Fix for #288.
Sort exercises within topics by difficulty (determined by x,y position in
sub-topic map).

Fix for #222
making this all work with ungrouped students
fixing undesired, debug-related cruft
Merge pull request #263 from bcipolli/save_the_django_serializers

Save the django serializers
Merge pull request #331 from bcipolli/fix_two_bugs

Fix two bugs: coachreports ignore ungrouped students, students can't see their own coachreports
Removal of unnecessary code in generating link URL.
Merge pull request #330 from rtibbles/tabular-link

Enable link alert and exercise sorting on tabular coach report view