Redmine integration should warn if issues_url is missing :id required parameter

Summary

When setting up Redmine Integration, 3 fields are required: Project url, Issues url, New issue url.

The second one expects the :id parameter to be present so that #32 can be transformed to http://redmine-url/issues/32 for instance. I discovered that by reading the documentation after my first failed test where I used http://redmine-url/issues as URL:

Note that the issues_url requires :id in the URL. This ID is used by GitLab as a placeholder to replace the issue number.

I think gitlab could raise a validation error (or warning?) when the required parameter is missing, this would avoid such error in the future.

Steps to reproduce

1. Configure Redmine integration on a project (https://gitlab.com/gitlab-org/gitlab/-/services/redmine/edit)
2. Configure issues_url without :id parameter and validate.

--> Everything is OK but Gitlab will fail to link #9494 to Redmine issue URL.

Example Project

Cf bagage/redmine-integration-bug!1

What is the current bug behavior?

It links to generic issues URL instead of pinpointing the exact reference.

What is the expected correct behavior?

It should warn user that the URL is missing the required :id parameter.