use callable for JSONField (#704)
use callable for JSONField (#704)

From the django warning,
```
eventkit_cloud.auth.OAuth.user_info: (postgres.E003) JSONField default should be a callable instead of an instance so that it's not shared between all field instances.
```
venicegeo/eventkit-cloud,terranodo/eventkit-cloud,terranodo/eventkit-cloud,terranodo/eventkit-cloud,venicegeo/eventkit-cloud,venicegeo/eventkit-cloud,venicegeo/eventkit-cloud,terranodo/eventkit-cloud,venicegeo/eventkit-cloud,venicegeo/eventkit-cloud