registered user URL upload

registered users may now upload files from URL (remote) locations; also
granted ability for registered users to "Proceed" when only single image
file is selected.
guaranteed filename uniqueness

anonymously uploaded filenames are now guaranteed to be unique. i.e., if
anon user uploads two *unique* files both named "ImageFile.jpg", the
first uploaded file will NOT be overwritten by the second (which could
potentially break the compare process)