refactor InstructorFeedbackUnpublishAction
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1806
clean up InstructorFeedbackUnpublishActionTest.java
Mentioned 'GitHub flow' in the process.md
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1806
Revert "clean up InstructorFeedbackUnpublishActionTest.java"

This reverts commit 13e34031b3958aa984e2e50bfd26d10c573a8d95.
for unpublishActionTest, I only test success and failure (due to accessibility). Other failure due to logic will be tested in the logic test
remove magic number in UnpublishActionTest
handle the case, when users unpublish an unpublished session
add coverage test for Unpublishing an unpublished session
Merge branch 'master' into Issue1806
rename test cases' names
put Unsuccessful Case handler into catch block, and re-use codes from logic
remove unused import
move normal path process into try block, with minor tweaks in statusToAdmin
Merge branch 'master' into Issue1806
better naming: session --> feedback session
Merge branch 'master' into Issue1806
refactor unsuccessful case handling in *unpublishAction
Merge branch 'master' into Issue1806