More Stylelint rules

What does this MR do?

• Extends the stylelint rules based on recommended rules for scss projects
• Fixes all problems that are currently reported as errors
• Following rules are still warnings (as the changes for those would pile up to much) but in the next iteration they should become errors as some of them are the most important ones: max-nesting-depth, scss/at-extend-no-missing-placeholder, selector-class-pattern, selector-max-compound-selectors (especially the nesting-depth and the compound selectors should also be lower)

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