Display referenced merge request statuses in the issue description
See gitlab-org/gitlab-ce#3180 for original issue. Previously discussed in !1941 (closed) and in !1771 (closed) before that.
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Show a "Related Merge Requests" list on the issue page under the Description as given by the UI mockup 
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Display all_references.merge_requestsandnote.all_references.merge_requests
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Consider displaying the merge request status in addition to the CI status to enable complete merge request status understanding.