Django-sendgrid-v5

Due to breaking changes in `sendgrid-python`'s newest v5.0.0 release: https://github.com/sendgrid/sendgrid-python/releases/tag/v5.0.0

Per this discussion: https://github.com/sendgrid/sendgrid-python/issues/328#issuecomment-321611488

Will unpin once `python-sendgrid` unpins its dependency to `python-http-client` (most likely in the v5.0.0 release)

Initial package release. More work needs to be done from an infrastructure perspective including adding more comprehensive tests.