Resolve "Fold URL references to files in GFM"

What does this MR do?

This feature allows for blob URLs to be folded into a shorter more descript one. See Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/14225 for more details on the ask

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

This is just for folding URLs for blobs. So the best places being the test generated would be to check out the text editors in the preview to make sure that they work.

Why was this MR needed?

This feature was requested by the github-ce team and was done in conjunction with a coding interview.

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #14225 (moved)