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
Fixed getter, modified tests.
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1810
Cover branch for invalidParametersException
Add action test for invalidParameters branch
Merge branch 'master' into Issue1810
[1810] Clean up InstructorFeedbackQuestionEditAction

fixes #1810