Prettier SCSS

What does this MR do?

This makes sure prettier runs through our {,S}CSS files in CI and locally.

A few things to note:

• It seems that function arguments and a few other things ignore the trailingComma setting, and are always stripped out. There's a related upstream bug about this.
• We set singleQuotes: true in our .prettierrc. I think double quotes are more common/natural in CSS, but in fact setting a workaround for SCSS files for this results in more changes, so I've not changed that.
• Some files needed adjustment after prettier was done with them to satisy stylelint. This is presumably why we don't run prettier on SCSS files in gitlab-org/gitlab. One stylelint rule had to be adjusted so that some files could satisfy both tools.

Screenshots or screen recordings

N/A

Integrations

• GitLab: mr_url
• CustomersDot: mr_url
• Duo UI: mr_url
• Status Page: mr_url
• Docs: mr_url
• Switchboard: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
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• The MR is added to a milestone.
• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update. More information can be found in doc/publishing-packages.md.

Components

• GitLab UI's contributing guidelines.
• If the MR changes a component's API, integration MR(s) have been opened (see integrations above).

Documentation

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• Content follows the GitLab Documentation Style Guide when appropriate.
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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

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• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.