removes get_or_none

Removes get_or_none base model classmethod, replacing instances with get_object_or_404, or with try/catch DoesNotExist, as appropriate.

Changes urls of submit_edit pages to take object_id as a url parameter rather than a post parameter.

Updates tests and templates accordingly.
This is going away by virtue of validation that will ensure a StageUrl has a non-blank url field right?