Migrate forum.nasqueron.org to Mailgun

Summary:
Mandrilapp discontinuated standalone service to integrate their
solution with Mailchimp marketing product. We switch to Mailgun.

Fixes T762.

Test Plan: Regenerate the configuration file on Dwellers

Reviewers: dereckson

Maniphest Tasks: T762

Differential Revision: http://devcentral.nasqueron.org/D367