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
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#23485 (moved)

Mentioned in issue #23485 (moved)

Looked at the code and checked out the branch. Looks good to me, so hopefully we won't find any weird CSS bugs come up!

Status changed to merged

Mentioned in commit 3b93574a

mentioned in issue gitlab#6675