Upgrade all python dependancies to the latest version.
Upgrades include major version changes and the upgrade to Django 2.2. May want squash commit.
Significant Django changes
- Add explicit on_delete for foreign keys
- Add on_delete for all foreign key migrations. Tested by running new migrations against an existing sample database from master.
## Related issues
- Closes #337 (closed)
- Closes #345 (closed)
- Closes #214 (closed)
Edited by Tom Mychost