[#6677] Account for duration for timed assessments to determine if grading can begin

[TheTallJerry]

Motivation and Context

This PR resolves #6677.

When determining when an assessment becomes gradable, the current code doesn't take into account the duration of timed assessments. A timed assessment that can be started between 2023-12-01 03:00 and 2023-12-04 03:00 with a duration of 2 hours, should be gradable after 2023-12-04 05:00, not after 2023-12-04 03:00.

Your Changes

Description:
Takes into account the duration of timed assessments when determining when they become gradable, evident from the flash messages displayed as mentioned in #6677.

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)
• Refactoring (internal change to codebase, without changing functionality)
• Test update (change that modifies or updates tests only)

Testing

Added unit tests:

• Timed assessments become gradable after due_date + duration
• Not timed assessments become gradable after due_date

Checklist

• I have performed a self-review of my own code.
• I have verified that the pre-commit.ci checks have passed.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have added tests for my changes.
• I have updated the Changelog.md file.

[coveralls]

Pull Request Test Coverage Report for Build 7062607185

• 14 of 14 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 91.882%

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Totals
Change from base Build 7047766771:	0.003%
Covered Lines:	38419
Relevant Lines:	41257

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💛 - Coveralls