This JSONBlob field type is a wrapper around Django's new generic
JSONField, but with the database column type forced to be text. This
should behave close enough to our old wrapper around
django-jsonfield's JSONField and will avoid needing to do the
out-of-band database migration.
The event_data field on event models, however, is getting an
overridden version that retains the underlying text data type for the
column, to avoid a heavy data migration on those tables.
Also, certain of the larger tables are getting these fields with the
NOT NULL constraint turned off, to avoid a long migration.
Remove the django.utils.six monkey patch we did at the beginning of
the upgrade.
