E2567. Review rubrics varying by round

[YuWang1925wy]

Assignment model:

1. Added a field vary_by_round
2. Edited the method num_review_rounds, so now it counts the number of due dates as the number of rounds
3. Edited the method varying_rubrics_by_round? Now it returns true only if per-round rubrics are enabled on the assignment and at least one attached questionnaire specifies a round via used_in_round

Assignment controller: Added vary_by_round to the strong-params whitelist

Due Date model: Added a class constant REVIEW_DEADLINE_TYPE_ID = 2 to determine whether a deadline is a review deadline

Added RSpec tests for updated methods mentioned above.

[github-actions]

	3 Warnings
⚠️	Pull request has duplicated commit messages.
⚠️	Schema changes detected without a corresponding DB migration.
⚠️	RSpec tests seem shallow (single it blocks or no context). Consider improving test structure.

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