Adds a "Final Source" field to the post replacement dialog. If
specified, the post's source field will be changed to this value after
replacement.
This makes fixing the source back to the HTML page after
replacement easier.
Bug: if a user replaces a post with another image, then replaces the
post back to the original image, then the deletion job for the original
image will still run. The will delete the original file, but that file
is now in use again and should not be deleted.
