CLC-4321 - Investigate possible issues with pending eGrades Export tool

https://jira.ets.berkeley.edu/jira/browse/CLC-4321

Provides validation and appropriate error display when state of course site not appropriate for eGrade Export. Ensures that export uses letter grade provided by Canvas via Grading standard/schemes, and only includes student grades from selected section.
@redconfetti The agreement we reached with the team was that the back-end would return camelCase keys for new properties. We agreed that we wouldn't replace all of them at once but definitely do it for the new ones. We already do this in many other parts of the CalCentral code and should do it here as well.