Allow instructor to control re-grading behaviour when test package is updated

[cysjonathan]

Issue

Whenever test package is updated, all past answers are regraded.

Requested behaviour

Instructor wishes to only re-grade latest answer, or not perform any regrading at all (e.g. until tested on their own submission).
We will present 3 options when a test case (with existing answers) is updated:

1. Do not run
2. Regrade only latest
3. Regrade all

Solution/Fix

Currently we can temporarily implement a dialog to allow the instructor to select the appropriate re-grading behaviour.

However, the current setup has the side effect that answer results that are not regraded become inconsistent with new test cases reflected on frontend.
Once implemented, we should create a separate issue to associate answers FK directly to test case(s), and implement a copy-on-write for test package updates so that past answers that were previously graded on old tests will still be consistent with their results. (This may require a rewrite of the test-package structure, to be investigated in the separate issue).