Add back ability support for `new_issue_url` for External Issue Trackers

Problem to solve

#44125 (closed) and !16353 (merged) removed support for adding new issues to external issue trackers from within Gitlab CE.

Further details

I am looking for integrating Git support into my Redmine. Currently looking at Gitolite and GiLab as main contenders. The lack of the ability to create issues from within GitLab is a major drawback to its feasibility.

Proposal

Add back support for (linking to) creating new issues in external issues trackers.

What does success look like, and how can we measure that?

Clicking on the Create Issue link will redirect to the URL defined by new_issue_url.

Links / references

https://docs.gitlab.com/ce/user/project/integrations/redmine.html