Allow mailers to be explicitly marked as inbound or outbound

Summary:
See PHI785. Ref T13164. In this case, an install wants to receive mail via Mailgun, but not configure it (DKIM + SPF) for outbound mail.

Allow individual mailers to be marked as not supporting inbound or outbound mail.

Test Plan:

• Added and ran unit tests.
• Went through some mail pathways locally, but I don't have every inbound/outbound configured so this isn't totally conclusive.
• Hit bin/mail send-test with a no-outbound mailer.
• I'll hold this until after the release cut so it can soak on secure for a bit.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19546