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