Rename unregisterFilter to deregisterFilter

fixes #2167
- Rename functions
- Adjust tests
Merge pull request #2168 from halfdan/deregister

Rename unregisterFilter to deregisterFilter
Cleanup dependency order in package.json

- because OCD
Ensure filters get default priority if priority is null

- null priority should be set to the default
- second part to https://github.com/TryGhost/Ghost-App/commit/86619cb087774a3e7cae39b13527191e62fdf5a4