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