Placeholders are hard to read due to their low contrast, and users might
want to read them after they're already gone, and so they cause both
accessibility and usability issues.
The imageable/documentable object is always the object the form builder
is based on; since we're already passing the form builder, we don't have
to pass the object as well.
The only exception are the poll answers. In this case, we're passing a
new answer as the object. That's OK; the same hack that we're using to
send the data to the answer URL without displaying existing attachments
causes the form to keep working the same way.
