Move Maniphest attached files section into textbox.
Summary: Add a button to the Remarkup area that explains how to attach an image and remove the separate upload field.
Test Plan: Check that the dialog pops up correctly and that dropping images onto the Remarkup area works.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin, AnhNhan
Maniphest Tasks: T879
Differential Revision: https://secure.phabricator.com/D6049