Add survey completion option for exercise

[ozer550]

Summary

• Adds an option for survey type for completion in exercise UI.
• add a feature flag for the new survey feature.
  

References

closes #4875

Reviewer guidance

• Enable the survey feature flag for one of the example users.
• Login with that user and create an exercise.
• Check that there is an option to set completion as survey.

[rtibbles]

Code changes all make sense. Confirmed that the value can be updated to survey and that this persists and is saved in the backend.