Draft: Allow embedding of code in Markdown

What does this MR do and why?

Todo

• Limit rendering of blob content
  • Only render when link href is the same as link text
  • Only render when link is at the end of line
  • Validate that the given start and end line numbers are correct
  • Only render when selected blob content is less than certain size or number of lines
  • Don't render blob content when inside lists, bold, italic, strikethrough
  • Skip binary blobs
  • Only render at most 3 (?) links per Markdown field
• Performance: Batch load projects / blobs
• Security
  • Redact blob content when user does not have access
  • Don't send blob content in email notifications when show_diff_preview_in_email is disabled. Or maybe just disable this in emails all the time as a first iteration.
• Frontend
  • Render syntax highlighted blob content with file information / SHA
  • Add project name if cross-project reference. Or maybe we can skip rendering in this case too?
  • Add max height and make it scrollable
  • Investigate why syntax highlighting colors are different from file viewer
  • Fix copy-paste behavior
  • Check if JS behavior is needed, like highlighting lines when line number is clicked

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How to set up and validate locally

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #300855