Switch from rails-sassc to cssbundling-rails

[david-yz-liu]

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.)

The old rails-sass gem is not receiving updates, and so this PR switches our processing of CSS files to use cssbundling-rails and Dart Sass, which is already installed through npm.

There are no user-facing changes, since this is just a change in build system.

I also took the opportunity to remove vendored copies of CSS files that could be imported directly from node_modules. I needed to add one gem-vendored CSS file, from cookies_eu to the repository due to rails/cssbundling-rails#81.

Screenshots of your changes (if applicable)

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

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

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)	X
🔧 Internal (change that only affects developers or continuous integration)

Checklist

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Before opening your pull request:

• I have performed a self-review of my changes.
  • Check that all changed files included in this pull request are intentional changes.
  • Check that all changes are relevant to the purpose of this pull request, as described above.
• I have added tests for my changes, if applicable.
  • This is required for all bug fixes and new features.
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Questions and Comments

(Include any questions or comments you have regarding your changes.)

[coveralls]

Pull Request Test Coverage Report for Build 9525014809

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 91.386%

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Totals
Change from base Build 9524684732:	0.002%
Covered Lines:	40052
Relevant Lines:	43147

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💛 - Coveralls