Assign tas final

[AinaMerch]

Proposed Changes

Add functionality to assign TAs to specific sections:
Add a new button to the graders page called "Assign by Section".
When the button is clicked, open a new modal that has a form that displays a list of all course sections, and next to each section is a dropdown with a list of TAs (plus an "empty" option). So, each section can be assigned at most one TA using this form. [If an instructor wants to assign multiple TAs to the same section, they'll need to submit the form multiple times.]
When the form is submitted, the controller assigns the TAs to all groups in their assigned sections. Note that all existing TA assignments remain unchanged; this form only adds new TA assignments.

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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)
🎨 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

(Include any questions or comments you have regarding your changes.)
I can't figure out how to get the right query to filter groupings by sections

[coveralls]

Pull Request Test Coverage Report for Build 10928790259

Details

• 86 of 113 (76.11%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 91.592%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app/controllers/graders_controller.rb	35	37	94.59%
app/assets/javascripts/Components/graders_manager.jsx	2	7	28.57%
app/assets/javascripts/Components/Modals/section_distribution_modal.js	2	22	9.09%

Files with Coverage Reduction	New Missed Lines	%
app/assets/javascripts/Components/graders_manager.jsx	1	37.4%

Totals
Change from base Build 10928761298:	-0.04%
Covered Lines:	40492
Relevant Lines:	43537

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💛 - Coveralls