Duplicate issue

Context

• ~"EE Premium"

Description

• Extension of and similar UI: https://gitlab.com/gitlab-org/gitlab-ee/issues/2001.
• Mark an issue (issue D) as a duplicate of another issue (issue I).
• The duplicate issue (D) is closed.
• The original issue (I) has UI to allow the user to navigate to the duplicate issue, in the relationships widget of #2001 (closed).

Outstanding questions

• Current description is focused on marking a duplicate and closing it.
• It implies a unidirectional relationship.
• Other use cases might favor maintaining both issues open. This is a bi-directional relationship.