added test cases for better coverage. changed method name to clarify its function
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1954
removed redundant FeedbackSessionType
added test cases to expand coverage

-> email task queue related test cases are in
FeedbackSessionEmailTaskQueueTest.java
added a TODO to remove an unused function
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1954
Rewrote comments for better clarity
Merge branch 'master' into Issue1954
[1954] Clean up FeedbackSessionsLogic

fixes #1954