remove course location from loc_mapper on delete
Remove duplicate code in transcripts_ajax.
Clean up references to answer distribution-like things

In the near future an answer distribution report will be displayed in the data download reports table.  In order to avoid confusion, we need to clean up several misleading labels and remove a redundant feature.

Note that the redundant feature is surrounded by a feature flag so that it can be disabled once the report goes live in the table.

Fixes: AN-694
Added image tags to the whitelist for matlab
Merge pull request #2985 from edx/zub/story/deletecoursefromlocmapper

remove course location from loc_mapper on delete
Merge pull request #2622 from edx/alex/fix_duplicate_code_in_transcripts_ajax

Remove duplicate code in transcripts_ajax.
Merge pull request #2972 from mulby/gabe/clarify-answer-dist

Clean up references to answer distribution-like things
Initial implementation for system_manager app
Renamed GroupExtension model to GroupRelationship