do not overwrite max_mark in grade summary

[pretendWhale]

Proposed Changes

(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)

There is currently a bug with display of extra marks on the grade summary page. When iterating over the collection of results, with percentage extra_mark's, a variable max_mark is set equal to the maximum mark for the current result. In the next loop iteration this value for max_mark will be used, and the current result's max_mark will not be used. This is fine when all results are for the same assignment, but leads to incorrectly calculated extra_mark's when the collection of results is from multiple assignments: only every other grade will be calculated correctly.

Screenshots of your changes (if applicable)

Associated [documentation repository](https://github.com/MarkUsProject/Wiki) pull request (if applicable)

Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)	x
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

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[coveralls]

Pull Request Test Coverage Report for Build 9665487788

Details

• 28 of 28 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 91.392%

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Totals
Change from base Build 9528555904:	0.006%
Covered Lines:	40079
Relevant Lines:	43174

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💛 - Coveralls