added entity_add_access task
added view/form/template/etc for add-access-file function/task
make_access_file complains if source file does not exist
make_access_file complains if can't create access file

Before, it was falling through this set of if-thens and returning
the default 'unknown' result in certain cases.
make_access_file uses only first layer of multilayer images

PDFs and certain TIFs can contain multiple layers.
Imagemagick accomodates this by creating multiple output files.
The DDR code didn't know what to do with multiple images, though,
which caused access file and thumbnail generation to fail.