Issue 1807
InstructorFeedbackQuestionAddAction
- It is currently possible to add questions with empty question text by modifying the js validator script.(instructorFeedback.js:64)

+ Added assumption to FeedbackAbstractQuestionDetails to assume questionText not empty.
+ Added test for empty questionText, expect assumption faliure.
+ Added test for non-existent enumerations of giver/recipient/question type, expect assumption faliure.
+ Added test for invalid questionNUmber
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1807
InstructorFeedbackQuestionEditAction
- missing else case, line 59

+ Add else case, test and assumption fail when editType is invalid.
+ Add test and assumption for invalid question number.
+ Add comment on when some parameters are null.
Remove catching and defining EntityDoesNotExistException since it is never thrown/not possible to be thrown by any method related.
Remove EntityDoesNotExistException since it is already caught by  function createFeedbackQuestion().
Added Assumption.assertNotEmpty/IsEmpty for String. Changed such assertions across TEAMMATES to use the functions.
undo some wrong changes
Added a modifyParamValue() function to make code in test/cases/ui/InstructorFeedbackQuestionAddActionTest.java clearer.
Added a TODO in FeedbackAbstractQuestionDetails to add a validate function.
Merge branch 'Issue1807' into Issue1810
Change some tests to use modifyParamValue()
FeedbackQuestionsLogic: Remove TODO because test is already done in the first test in function.
InstructorFeedbackEditPageUiTest
+ Added getter for maxNumScaleBox in InstructorFeedbackEditPage
+ Added test for numscale validation to check javascript.
+ Added TODO for numscale, currently, user is able to submit when min/max/step is empty and trigger an error page.
+ Added some TODOs for mcq/msq
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1810
Refactor InstructorCourseDeleteAction
Fixed getter, modified tests.
Add comment for testing access
Change test case due to change the action result from ShowPageResult to RedirectResult
Refactor, delete redundant test
Add test case for getCoursesForInstructor
Fix comment
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1810