use sendgrid for sending emails

https://assembly.com/formspree/bounties/5

---

Sendgrid [doesn't support sending from our own domain](https://sendgrid.com/docs/User_Guide/whitelabel_wizard.html) for accounts lower than the Silver plan, so we can use the `DEFAULT_SENDER` we're already using and no DNS configurations have to take place now, the problem is that the emails will be sent in fact with a sendgrid domain as the original verified sender and Gmail (maybe others) will display our emails with a nice `via sendgrid.info`.

For generating an API user and key, go to https://sendgrid.com/credentials