Clean up InstructorFeedbackUnpublishAction #1806

this fixed #1806 

Commits:
1. refactor InstructorFeedbackUnpublishAction
2. only test success and failure (due to accessibility and 'unpublishable') in the InstructorFeedbackUnpublishActionTest
3. remove magic number in the InstructorFeedbackUnpublishActionTest
4. handle the case, when users unpublish an unpublished session (coverage test included in 2.)