Make rich blob viewer wider for PC

What does this MR do?

Unifies margins in the rich blob viewer to remove weird margins around itself for PC.

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

Margin 16px is good enough?

Why was this MR needed?

Rich blob viewer has weird margins around itself for PC. See #41709 (closed) for some screenshots.

Screenshots (if relevant)

The following screenshots were taken in the 874px-wide environment (= md).

README.md	blob view of a Markdown file

The following screenshots were taken in the 992px-wide environment (= md). (Note that !16181 (merged) will fix a bug occurring on wider environments than 790px (= some of md and any of lg).)

README.md	blob view of a Markdown file

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered

What are the relevant issue numbers?

Closes #41709 (closed)

• Ongoing mitigation for README on project top: !16181 (merged)
• Previous mitigation for mobile: !14011 (merged) (#37368 (closed))
• Root cause: !12198 (merged) (#31890 (closed))