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