Support EU domains for Mailgun API
Summary:
See https://discourse.phabricator-community.org/t/mailgun-eu-zone-not-working/2332/4.
Mailgun has a couple of API domains depending on where your account is based.
Test Plan:
- Sent normal mail successfully with my non-EU account.
- Waiting on user confirmation that this works in the EU.
Reviewers: amckinley
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20055