CLC-4487 - Resolve issue with CanvasCourseGradeExportController#is_official_course

https://jira.ets.berkeley.edu/jira/browse/CLC-4487

In last PR the #is_official_course method required the canvas_course_id in the session instead of the parameter. Also, the Canvas::Egrades class requires the user_id when it's not necessary. See [comments](https://jira.ets.berkeley.edu/jira/browse/CLC-4487?focusedCommentId=186810&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-186810) for notes on why it's not really necessary.

Now #is_official_course can truly respond to non-authenticated public requests.