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.
Yes, I noticed this with a DataC PR too. I now know to look out for Github warn icon.