Wrap `json.load` into an exception handler so we don't get exception if file is non-existent.
Merge branch 'nalanda-rct2' into feature/2122-Exam-list-page

Conflicts:
	kalite/student_testing/views.py
Merge branch 'nalanda-rct2' into feature/2122-Exam-list-page
Initial work for #2122 - Exam list page - now displays the list of tests which checks the actions for teacher or student.
TODO:
1. set exam mode on per test
2. redirect student to exam if exam mode is on
Update the TODO list for #2122.
Sort list of tests by title.  Have buttons with appropriate classes when a test is in exam mode or not.
Capture the click event for the button to be sent to the server.
Toggle value of Settings.EXAM_MODE_ON by an admin user from list of tests page at `/test/list/`.
Redirect student to exam page of test set for exam-mode on Settings.
Add "Tests" navigation link for admins which shows the test list page.
Show an `alert` dialog if a test cannot be set to exam-mode.
Bug fix related to #2122 to show page 404 if user is student and the test page being accessed is not the same as the Settings.EXAM_MODE_ON value.
Merge pull request #2144 from cpauya/feature/2122-Exam-list-page

Feature/2122 exam list page