Adding A/B/C testing to hint generation.
Fixes for test cases.
Merge from finished.
Updated based on Ellen's comment.
Updated get_best_next_hint to get_third_party_hint.
Merge branch 'delivered'
Drop hints table, add source column to level_source_hints.
Update Stanford hint importing to changed schema.
Changed hints editor to only show crowdsourced hints.
Factored out string constant LevelSourceHint::CROWDSOURCED.
Merge pull request #838 from code-dot-org/delivered

Deploy to production
Merge branch 'delivered'
Merge pull request #849 from code-dot-org/delivered

Deploy to production
Merge pull request #852 from code-dot-org/delivered

Deploy to production
Changes related to retrieving crowdsourced and external hints from database.
- Added tests to level_source_test.rb.
- Moved is_experimenting_feedback? from activity_hint.rb to experiment_activity.rb.
- Moved code to retrieve hints from application_controller.rb to level_source.rb.
Minor changes to comments and formatting.
Added missing newline at end of file.
Merge branch 'master' into feedback_collect

Conflicts:
	config/stanford-hint-messages.tsv
	db/schema.rb
	lib/tasks/seed.rake
Incorporated reviewer feedback.
Addressed reviewer comments.