Prevent delivery of email to disabled objects

Summary: See T625. Facebook's REST-based MTA layer had a check for this so I
overlooked it in porting it out. We should not attempt to deliver email to
disabled users.

Test Plan:
Used MetaMTA console to send email to:

• No users: received "no To" exception.
• A disabled user: received "all To disabled" exception.
• A valid user: received email.
• A valid user and a disabled user: received email to valid user only.

(Note that you can't easily send to disabled users directly since they don't
appear in the typeahead, but you can prefill it and then disable the user by
hitting "Send".)

Reviewers: btrahan, jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: skrul, aran, epriestley

Differential Revision: 1120