Switch to Postmark for email delivery

[camdendotlol]

Summary

• switches from using Sendgrid via SMTP to Postmark via the official postmark-rails gem, which uses the Postmark API

Using the Postmark API rather than SMTP will provide more clarity if/when email delivery issues occur in the future.

Closes #462

To do

We still need to add covecollective.org as an authorized sender on Postmark.

Testing

1. Go to the staging site (let me know first so I can deploy this branch to staging)
2. Invite some users to an anthology with email addresses that link to your Performant email using the + trick
3. The invitation emails should arrive in your inbox

[camdendotlol]

@jamiefolsom This has passed Steve's testing, but we still need info@covecollective.org to be added to the verified senders list. As far as I can tell, I don't have the permissions required to do that in Postmark.