R1:690a460c8e8d

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…