Fix to extra SyncSession deletion.
Merge pull request #2091 from learningequality/release-0.12.0

KA Lite release 0.12.0
update version info
Fix to our JSON serializer to prevent error ('first' attribute missing).
Merge branch 'master' of github.com:learningequality/ka-lite into release-0.12.0
Fix full_clean (duplicate def on base, and missing kwargs on Device).
Allow Device signed by central server, so they can be modified centrally.
Merge branch 'release-0.12.0' into central-ecosystem-merge
Update version.py
Merge pull request #2209 from learningequality/release-0.12.0

Release 0.12.0
fix version.py comment to make it work for windows
Don't add distributed to INSTALLED_APPS within updates app.
Typo correction.
Enforce group and facility name uniqueness at form-level, not DB-level.
Drop the unique_together constraint on FacilityUser username/facility.
Since we now allow duplicate usernames, attempt login for each in turn.
Simplify FacilityUserForm validation, and also make it more lax.
Update tests to allow duplicate names for users/groups/facilities.
When logging in, try a case-sensitive match before going case-insensitive.
When deleting group, manually loop through users to remove from group.