Resolve "Ability to download single file"

What does this MR do?

It adds a new feature whereas a single file in a repository can be downloaded.

Still a WIP since I am not yet able to handle the case whereas the file to be downloaded is a dotfile. The file extension is being changed to txt. Edit: removed WIP since I do not think it's possible to solve that one.

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

I am using the download attribute of the a tag which is currently not supported by some browsers. However, I found that the download for the whole repository is using it as well so I continued with this.

Why was this MR needed?

Resolves #23705 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #23705 (closed)