Improve SendGrid integration

Summary:
We want to allow at docker run time to declare the SendGrid config.

New environment variables:

• PHABRICATOR_USE_SENDGRID: if set, will use SendGrid as mail adapter
• PHABRICATOR_SENDGRID_APIUSER: the API username to access SendGrid
• PHABRICATOR_SENDGRID_APIKEY: the API key to access SendGrind

Fixes T576.

Test Plan: Tested with Zed Phabricator container.

Reviewers: dereckson

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images, Zed

Maniphest Tasks: T576

Differential Revision: http://devcentral.nasqueron.org/D68