core_files: Add MIME types for iWorks files, update various groups

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
assignsubmission_file: Don't print 'other' if that's all there is

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
assignsubmission_file: Turn off 'restricttypes' if types list is empty

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
experiment in using a custom form control

Instead of jury-rigging a whole lot of individual checkbox elements
in a group using CSS to produce the appearance of a heirarchy, this
uses a custom form element that encapsulates all the setup and
validation.