Include 80-column marker on the right margin for code snippets
Description
Part of code review involves checking code against style guides, which typically include recommendations on maximum line length. Code listings throughout GitLab currently give no indication of line length.
Proposal
Include a right-hand margin line at the 80-character position in code listings.
Links / references
Here's how the line looks in the GitLab UI:
And here it is in a native editor: