Change GradebookEntry key to remove volatility

The key to GradebookEntry (https://schema.ed-fi.org/datahandbook-v320c/#/GradebookEntry560) has both a teacher-submitted string and a date, both of which will almost certainly be volatile in field implementations. a new key structure is needed.

This point was made and validated by the LMS Data and Analytics SIG, which looked at this issue. See notes on https://techdocs.ed-fi.org/display/ESIG/Meeting+3+-+2021-06-24 

(Note: this key structure is the legacy of Ed-Fi 1.0, when nightly wipe-and-replace syncs of data were the norm and such keys were not problematic because they could be updated daily).