Make packages mailable and subscribable

Summary:
Ref T10939. Fixes T7834.

• Make packages into mailable objects, like projects and users.
• Packages resolve recipients by resolving project and user owners into recipients.

Test Plan:

• Added a comment to a revision with a package subscriber.
• Used bin/mail show-outbound to see that owners got mail.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7834, T10939

Differential Revision: https://secure.phabricator.com/D15912