Only show the drag and drop indicator when the review request is editable.

If a user wasn't logged in, or they didn't own a review request, they would
still see some visual feedback when dragging files over their browser window
inviting them to upload the file. This change fixes that.

Testing Done:
Ran JS tests.

Tried to drag-and-drop a file:
  as the creator of the review request - indicator appears
  while signed in as an admin - indicator appears
  while signed in as a different user (not admin) - indicator does not appear
  while not signed in - indicator does not appear

Bugs closed: 3542

Reviewed at https://reviews.reviewboard.org/r/6312/