Keep previous results after a newer revision is collected for a grouping

[mishaschwartz]

If a submission has an associated result (that may or may not have been marked, annotated, etc.) and a new submission is created for that grouping (recollected), keep the original submission and associated result around and provide the option to return to it by collecting the previous revision again.

For example:

• revision A is collected, and partially marked
• the student submits a new revision
• revision B is collected
• the instructor decides to grade submission A (not B)
• the instructor collects submission A again and continues marking from where they left off

[david-yz-liu]

Resolved in #7256