Stylelints for duplicate selectors and using BS4 classes

What does this MR do?

Introduces 2 GitLab specific style linters:

stylelint-gitlab/duplicate-selectors for detecting selectors which have 1:1 the same css props + values.

stylelint-gitlab/utility-classes which checks props + values against our BS4 utility classes.

It is also easier now to run stylelint against a single file on the command line with

yarn run stylelint-file app/assets/stylesheets/pages/projects.scss

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer