Enable SingleLinePerSelector in scss-lint
## What does this MR do?
Enables `SingleLinePerSelector` in `.scss-lint.yml`
## Are there points in the code the reviewer needs to double check?
This MR touches a lot of files, just need another pair of eyes to look through and make sure I didn't break anything.
## Why was this MR needed?
Improve SCSS maintainability and consistency
## Screenshots (if relevant)
None
## Does this MR meet the acceptance criteria?
- Tests
- [x] Added for this feature/bug
- [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
## What are the relevant issue numbers?
#23485
See merge request !7081
- Add match line header to expected result for `File#sections`.
- Lowercase CSS colours.
- Remove unused `diff_refs` keyword argument.
- Rename `parent` -> `parent_file`, to be more explicit.
- Skip an iteration when highlighting.
