Integrate external issue tracker with entire GitLab instance for merge requests

Currently all our external issue tracker integrations (including the custom one, https://docs.gitlab.com/ee/user/project/integrations/custom_issue_tracker.html), are at the GitLab project level. I.e. you configure the integration in a GitLab's project settings, and then integration is established only for GitLab merge requests in that GitLab project.

This issue is to have a single configuration at the entire GitLab instance level, so that once you make that one configuration, it would apply to all merge requests in all projects in the GitLab instance.