Upgrade Sendgrid to the modern mailer API; removes "api-user" option

Summary:
Ref T920. Ref T5969.

• Update to the new "$message" API.
• Update to Sendgrid v3.
• Add a timeout.
• This removes the "api-user" option, which Sendgrid no longer seems to use.

Test Plan: Sent Sendgrid messages with bin/mail send-test ... using subject/headers/attachments/html/to/cc.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: jbrownEP

Maniphest Tasks: T5969, T920

Differential Revision: https://secure.phabricator.com/D19960