Django-zengo

- Removed the use of a transaction wrapping event processing, as this was causing very long-running transactions for some projects. This shouldn't have a significant impact and error reporting via the `logging` module should be relied upon.
- Added a "processed OK" filter to the event admin (thanks myles)

Some tickets have an undefined priority, which was not being handled properly.

Adding support for Django3 and dropping support for Python2

Zendesk authors comments with a special user with ID -1. This update allows Zengo to sync comments made by this special user.