Detect full issue URL in RelatedIssueFinder

What does this MR do and why?

This will allow to keep Danger happy by using full issue URLs instead of getting the following error:

- [ ] ff965449227c20a97068e894d428722c2a51785c: Use full URLs instead of short references (`gitlab-org/gitlab#123` or `!123`), as short references are displayed as plain text outside of GitLab. For more information, take a look at our [Commit message guidelines](https://docs.gitlab.com/ee/development/contributing/merge_request_workflow.html#commit-messages-guidelines).

I'm re-adding the full issue URL in gitlab-org/gitlab!149079 (merged), so that the AddQuarantineLabelsToRelatedIssue processor can add the quarantine & quarantineflaky labels to the related issue once the quarantine MR is merged.