Conpherence - make attaching files work better

Summary: unifies the code and presentation between adding files via email and web. Also makes it possible to "attach" the same file multiple times, either by just talking about it in the different messages or multiple times in the same message.

Test Plan: sent message with attachment - it worked! sent a message referencing previous attachment - it work! sent a message with the same attachment in it like 12 times - it worked!

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T2399

Differential Revision: https://secure.phabricator.com/D4679