Update Postmark adapter for multiple mail media
- Update the inbound whitelist.
- Add a little support for media configuration.
- Add a service call timeout (see T5969).
- Drop the needless word "Implementation" from the Adapter class tree. I could call these "Mailers" instead of "Adapters", but then we get "PhabricatorMailMailer" which feels questionable.
Test Plan: Used bin/mail send-test to send mail via Postmark with various options (mulitple recipients, text vs html, attachments).
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D19956