Django-sendgrid-v5

1) Adds ASM and suppression group support
2) Strips brackets from content-id since they are added back by sendgrid's api

Now supports Sendgrid's sandbox mode in DEBUG mode (by default).

https://sendgrid.com/docs/Classroom/Send/v3_Mail_Send/sandbox_mode.html

Thanks to Fandekasp for the code review and feature request

Fix per issue 2

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)