Bugfix: Initialize new tabname to "Empty"

In Studio, when the course administrator creates a new tab, its name was being
initialized as null in the mongo database. This caused students in LMS to get a
server error when trying to access the course page, since LMS was trying to
process the null tabname for translation, etc. Now tabname is initialized as
'Empty', which is how it was being done in the frontend.

STUD-2000
Use a version of opaque keys that identifies version numbers in the key prefixes
Make test_export_all_courses do not rely on the order in which files are returned by os.walk().
Disable failing test STUD-2003
Merge pull request #4546 from edx/zoldak/disable-test

Disable failing test STUD-2003
Merge pull request #4474 from Stanford-Online/sjang92/null_tabname_bugfix

Bugfix: New Tab name being initialized as null
Merge pull request #4543 from edx/tim/test_export_all_courses-was-flaky

Make test_export_all_courses not flaky
Merge pull request #4537 from cpennington/new-locator-key-prefixes

Use a version of opaque keys that identifies version numbers in the key prefixes
BLD-1104: List units that use group configuration.
REVERT THIS COMMIT BEFORE MERGE.
Addressed some comments.
Fix errors tests.
Fix tests.
Get unit safely.
Update tests.
Addressed more comments.