Filing GitLab issues for merge requests

[Ralph Seichter]

I'd like to suggest that a GitLab issue should be filed for our merge
requests, and that merge requests should link to at least one issue.

This would make it easier for non-contributors who wish to file any type
of issue to see if somebody has already worked on that aspect. In my
experience, this can avoid duplicates. From what I see, users with less
technical aptitude tend to look more at the existing issues than at
merge requests. Also, the issue can be used for discussions with non-
contributors, leaving the pull request conversations more focused on
technical aspects.

See also:

  https://about.gitlab.com/2016/03/03/start-with-an-issue/
  https://about.gitlab.com/2016/03/08/gitlab-tutorial-its-all-connected/

-Ralph