Switch over to file attachments entirely in place of screenshot uploading.

This removes the ability to upload screenshots, and instead moves to
using file attachments for everything.

Dragging-and-dropping a file over the review request will just have a
big "Drop to Upload" link, instead of "Drop Screenshot" and
"Drop File Attachment."

The "Upload Screenshot" link on a review request is gone.

The entry for a non-repository review request now says
"(None - File attachments only)" instead of "(None - Graphics only)".

Tested that uploading an image went through the file attachment mode only.

Verified that all public ways of uploading a screenshot were removed.

Verified that the New Review Request form now says "File attachments only"
and not "Graphics only".