Grading should update all at once, not piecemeal

[sahi1l]

Describe the bug When I go from one student to the next in the Grading panel, the different parts of the problem—the code, the history loading, and the grade and comments at the bottom—update at different times, so that it's hard to know when the updating is done. This is particularly frustrating when I want to go through several students quickly. Ideally it would update instantaneously, but perhaps you can hide the panel until it is completely updated, or add an indicator that says "Please Wait" until the update is complete, something like that.

To Reproduce Steps to reproduce the behavior:

1. https://runestone.academy/runestone/admin/grading
2. Choose an assignment, then a question, then the first student
3. Click "save and next"
4. Watch the panel on the right update.

[sahi1l]

It's also currently taking 6-12 seconds for each assignment to pop up, which is almost literally painful.