tracking user's attempt in every play

- add fk from UserAnswer to UserAttempt
- add is_completed and completed_at fields in UserAttempt
- auto fill old UserAnswer with correct UserAttempt
better display for UserAttempt
latest_for_user should check is_finished

no need to fetch finished ones
track UserAnswer's UserAttempt
track UserAttempt bad request and update finished_at, is_completed, and completed_at when it's complete
set all UserAttempt to finished when the user is playing new lab
check for UserAttempt before checking UserSave
whitespace and stuff
Display results and correct answers in a page in edX after finishing lab
better display for result page
Merge branch 'labster/feature/result-page' into labster/master
add IsExplorable