ENH: move image upload Form to rightsidebar

Reasons:
Looks out of world below the submission form in new-item.html.

Future:
Currently we rely on Restructured Text. Can probably use PageDown but need
to shift to Markdown format.
MAINT: improve image upload message
BUG: add media_root for image uploads in debug mode

1. in debug=true mode, images don't get uploaded unless media root is present