Declare OpenDKIM as milter for Postfix

Summary: Ref T2066

Test Plan:
This needs to be deployed carefully:

• Check Postfix logs
• Send mail for each domain to check-auth@verifier.port25.com

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2066

Differential Revision: https://devcentral.nasqueron.org/D3553