refactor *FeedbackResultsDownloadAction
clean up *feedbackResultsDownloadActionTest
optimize verifyAssumptionFailure logic
Revert "optimize verifyAssumptionFailure logic"

This reverts commit 14dbedfb9343c639aa25286e81c2bd23d4e085e4.
Merge branch 'master' into Issue1837
add fileName and fileContent tests to ensure it's downloadable
edited/added test cases to improve coverage
removed a TODO

Parameters not expected to be null and even if they are, no damage is
expected.
defense against null parameter

assertion to check for null parameter
add test cases for updateSession
improved validation checks in IFESA

now, even if malicious text comes via graceperiod/timezone, they will be
ignored
more thorough checks of parameter values
Merge branch 'master' into Issue1837
add test cases for combineDateTime

removed redundant test cases
improved function to handle exceptional cases
debugged convertToDate
missed a possible exception IFESA
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1854
make dev green
minor changes in coding format/std.
[1837] Clean up InstructorFeedbackResultsDownloadAction

fixes #1837
Merge branch 'master' into Issue1854