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.