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.
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.