Add a "mailable" function datasource

Summary: Ref T4100. Add a function datasource for filtering mailable objects (e.g., subscribers).

Test Plan: Searched for objects with "Current Viewer", "Members: Dog Project", etc., as a subscriber in global search.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12524