Display referenced merge request statuses in the issue description

See gitlab-org/gitlab-ce#3180 for original issue.
Previously discussed in !1941 and in !1771 before that.

* [x] Show a "Related Merge Requests" list on the issue page under the Description as given by the UI mockup
* [x] Display `all_references.merge_requests` and `note.all_references.merge_requests` 
* [x] Consider displaying the merge request status in addition to the CI status to enable complete merge request status understanding.

See merge request !1991