Added Stylelint Setup

What does this MR do?

It adds stylelint and specifically stylelint-scss to our CI pipeline. These are now basic recommended rules activated to get going and then activate iteratively more and more rules and even our own specific ones. Based on the basic rules some css adaptions were already done.

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