Sendgrid migration

[rickr]

Our sendgrid account is now setup. To complete this we will need to:

• [ ] Migrate our templates from mailchimp to sendgrid ~- [ ] Migrate all of our contacts from mailchimp to sendgrid~
• [x] Add all users in our database to sendgrid
• [ ] Transfer unsubscribes to sendgrid
• [x] Create whitelist DNS records
• [ ] Verify the backend is sending transactional email via the operationcode-transactional account operationcode/operationcode_backend#51
• [x] Create a job that adds users to sendgrid's contacts operationcode/operationcode_backend#49
• [ ] Remove the mailchimp job on signup operationcode/operationcode_backend#50

[hollomancer]

I think it might be better to migrate our contacts from the user database to Sendgrid, rather than Mailchimp to Sendgrid. Given the large differential in our total entries between Mailchimp and Sendgrid, it's likely many have gotten lost.