an original must be unique to a resource
add kind column to allow model to return subclass
add a foreign key to an unique original
add file probes
use CTI kind column
add #itemless to dataset
update associations and #resourceless to reflect reversed original-resource association
move #associated_dataset to class
add .associated_class
add .pk_format
add #table_name
rename mp4 to m4v
add png converter
rename block variable
scope #itemless to collection
add ConverterRole, along with thumbnail and poster methods