Add validation check for mismatched "Grade As" settings in Subsections

[ormsbee]

(edited by @bradenmacdonald:)

If a unit is configured ("Grade As") as a certain assignment type (e.g. "Homework"), but that assignment type doesn't currently exist in the course's grading policy, an error should be displayed in Studio.

^ Like this but it should say

This subsection is configured as "[type]", which doesn't exist in the current grading policy.