Add an easy way to attach a link to the file of the comment message
Following up from !1164 (comment 845614078)
When we post a comment as @gitlab-bot, we sometimes attach a message trailer to link to why the message was generated, and where to improve the message like below:
This message was generated automatically. You're welcome to improve it.
This also applies to messages posted by policies:
You are welcome to help [improve this comment](#{ENV['CI_PROJECT_URL']}/blob/#{ENV['CI_COMMIT_SHA']}/policies/stale-issues.yml).
It would be great if we can add an easy way to attach this so we don't forget to add it, and also it would help if we can use the same language for the same purpose.
This applies to:
- A policy file which is defined using YAML, should link to the policy file and potentially the CI job running it (it will help for debugging as well)
- A reactive message which is usually defined in a Ruby file, should link to the Ruby file defining the message. This does not run in CI so there's no CI job for it.