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.
Complete conversion of Test API resource to using PK instead of title.
Use file basename as PK.
Fix Test API Resource to allow proper generation of Resource URI.
Change all server side code to process test_id instead of test_title.
Fix client side code to query based on test_id.
Remove colon from "Begin Test:" button.
Merge pull request #2144 from cpauya/feature/2122-Exam-list-page

Feature/2122 exam list page
Merge branch 'nalanda-rct2' of github.com:learningequality/ka-lite into nalanda-rct2

Conflicts:
	kalite/student_testing/api_resources.py
	kalite/student_testing/urls.py
clear any preexisting logs when we initiate the update process
Update the BUILD_HASH whenever we start the server