Conpherence - make updating title + picture a dialogue that uses drag and drop upload control

Summary: decent title. Stylistically its probably a bit rough. Also, I think @chad describes an even hotter workflow in T2418. Note this removes the "default image" thing which I don't think makes sense conceptually since by default the image changes to who replied last...

Test Plan: uploaded an image - worked. uploaded a txt file - got ugly errors that file was not supported.

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T2417

Differential Revision: https://secure.phabricator.com/D4668