Improve Mailgun integration

Environment variables kept:

• PHABRICATOR_USE_MAILGUN: if set, enable Mailgun as default mailer
• PHABRICATOR_DOMAIN: if set, use it as Mailgun domain

New environment variable:

• PHABRICATOR_MAILGUN_APIKEY: if set, store it in the configuration

You can so get a fully functional Mailgun integration with these three parameters
to your docker run line:

-e PHABRICATOR_USE_MAILGUN 1 \
-e PHABRICATOR_DOMAIN phabricator.domain.tld
-e PHABRICATOR_MAILGUN_APIKEY secretapikey