Show profile pictures in subscribers dialog

Summary: Ref T4400. Also stops rendering "and 1 other" in subscriber lists, since it looks a bit silly in practice (we can just put the other subscriber there instead). Don't do the "and x others" until X is at least 2.

Test Plan: Viewed/clicked subscriber lists and transactions.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4400

Differential Revision: https://secure.phabricator.com/D8573