R1:64e3296fe682

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…