Improve output of /debug/jobs/list

What does this MR do?

Improves output of /debug/jobs/list with adding \n at the end of each line (currently the last line is missing EOL character) and simplifying writing to ResponseWriter.

Why was this MR needed?

Please read gitlab-cookbooks/cookbook-wrapper-gitlab-runner#2 (closed) for a reference.

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

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Fixes gitlab-cookbooks/cookbook-wrapper-gitlab-runner#2 (closed)