Apply responsive note styles to parallel diffs

What does this MR do?

• Apply responsive note styles to parallel diffs
  • Add notes-media mixin that inserts a side by side CSS rule that applies to notes in a parallel layout.

Inspired while working on https://gitlab.com/gitlab-org/gitlab-ce/issues/32801

Are there points in the code the reviewer needs to double check?

• Where should the notes-media mixin live?

Why was this MR needed?

• We only had a few response bare-necessities applied to parallel diff notes compared to normal notes. Things kicked in way too late given they are side by side.

Screenshots (if relevant)

.	Screenshot
Before
After

Does this MR meet the acceptance criteria?

• Tests
  • 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?