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.
Update version.py
Fix to chronograph admin templates to get admin pages loading again.
Merge branch 'release-0.12.0' into central-master