Fixed bug.
Began refactoring.  Not yet tested.
Began major reorg.  Still undecided.
Work in progress for discussion.
Merge branch 'optional-hints2' of github.com:code-dot-org/dashboard into optional-hints2

Conflicts:
	app/models/experiment_activity.rb
Further progress.  rake test passes.
Minor changes.
Fixed bug.
Fixed tests to work with imported Stanford hints.
Added omitted file.
Incorporated Laurel's excellent feedback.
Removed unnecessary 'private' keywords.
Added necessary migrations.
Moved code into activity_hint.rb, added tests.
Made improvement suggested by Laurel.
Merge pull request #1065 from code-dot-org/optional-hints2

Refactoring and additions related to hint experiments