Merge pull request #2287 from learningequality/nalanda-rct2

Nalanda rct2 into central-master
Safely continue if deleting default_language fails before creating again.
Merge branch 'nalanda-rct2' into central-master
Directly import sha_constructor from hashlib to avoid Django warning.
Add software update link to overview page.
Tweak style and wording on software update page.
in coachreports/urls.py: use \w instead of \d for getting the test_id
in coachreports/urls.py: use \w instead of \d for getting the test_id
Display more details about sync job status on device page.
Merge branch 'nalanda-rct2' of github.com:learningequality/ka-lite into nalanda-rct2
Merge pull request #2296 from learningequality/central-master

A few changes from central-master
Methods to gracefully close content views, and use in playlist nav.
Don't try to close exercise_view if it doesn't exist on quiz.
Reliably suppress feedback on submit button in test and quiz modes.
Turn exercise answer button orange even when not logged in.
Fix bug where initial status message wasn't shown on exercises.
Merge branch 'nalanda' of github.com:learningequality/ka-lite into nalanda-rct2
Properly pass context_id and context_type in for attemptlogs etc.
Merge branch 'fix_exercise_logging' into nalanda-rct2
Merge branch 'playlist_exercise_fixes' into nalanda-rct2