Move the business logic of which passes are issuable to model.
Allow holder_jpeg to be NULL.

- Do not attempt to issue passes if there isn't a photo.
- Have the JPEG controller handle a null photo, and redirect to a new
  missing photo svg file.

Missing.svg is from
http://commons.wikimedia.org/wiki/File:Gnome-stock_person.svg and is
licensed under GPLv2 or later. We're GPLv3, so that's OK.