Grades and Gradebook work - Ed Ticket

This epic brings together a set of cohesive work to update the gradebook domain and its associated model for grades based upon the following:

• Address the backlog of tickets in this area (see below)
• Address the use case of needing to display a student's current/interim grade before the end of a grading period
• Develop a model with the flexibility to bring in data from SIS gradebooks but also third-party gradebook apps
• Unify the LMS Toolkit Design work with Assignment and Assignment Submission
• Include the ability to incorporate online assignments from various learning resources
• Address other use cases obtained from the community

Relevant tickets include:
DATASTD-1599 Change GradebookEntry key to remove volatility
DATASTD-1646 Capture concept of "current" grade
DATASTD-1638 Support Standards-Based Assignments: Optionally Associate StudentGradebookEntry with LearningStandard