Create User class method to retrieve orphaned users

[donny-wong]

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.)

We would like to have a feature to remove users on MarkUs who are not attached to any course. This PR is the first step of that task, which is to retrieve orphaned users.

Screenshots of your changes (if applicable)

Associated documentation repository 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)	x
🐛 Bug fix (non-breaking change that fixes an issue)
🎨 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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Questions and Comments

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[coveralls]

Pull Request Test Coverage Report for Build 16893628941

Details

• 18 of 18 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 91.884%

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Totals
Change from base Build 16886073756:	0.003%
Covered Lines:	42092
Relevant Lines:	45107

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💛 - Coveralls

[david-yz-liu]

@donny-wong nice work, I just left one comment about the testing