R1:dbedb012eb7e

Add support for SendGrid as an outbound mail adapter

Summary: SendGrid is a popular mail delivery platform, similar to Amazon SES. Provide support for delivering email via their REST API.

Test Plan: Created a SendGrid account, configured my local install to use it, sent some mail, received mail.

Reviewers: tuomaspelkonen, jungejason, aran

CC: ccheever

Differential Revision: 347