User with custom role + read_code can view MD link

What does this MR do and why?

• Previously, links were broken because the download_code ability was checked

Screenshots or screen recordings

Before (viewing private project README as custom guest user with read_code):

After (viewing private project README as custom guest user with read_code):

How to set up and validate locally

• To reproduce:
  • As an admin, create a custom role with read_code for a group: https://docs.gitlab.com/ee/user/permissions.html#custom-roles
  • Add a private project to that group
  • Add an image to the private repo's README or other markdown file
  • Add a user to the private project with the custom role
  • Before this MR: image link is broken
  • After this MR: image link works as expected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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