Make "New Conversation" a workflow dialog in Conpherence

Summary: Ref T2416. My actual goal here is to stop having two places where we call the "load and display a list of threads" logic, but this is generally weird and no longer necessary with drag-and-drop-to-upload-remarkup.

Test Plan: {F37992}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2416

Differential Revision: https://secure.phabricator.com/D5475