Support "Closing as Duplicate of" & "Mark as Duplicate of" github Issues

[ShareASmile]

FR: Mark and Close issues as a duplicate of another issue

In #1373 ability to specify a reason was added but github now has floated a new useful option "Mark issues as a duplicate" and tried to remove unnecessary burden for both maintainers and collaborators, i.e. specify a new reason for closure of an issue "duplicate of another issue" when closing an issue by tapping the "Close" action in the app bar.

As of now Github now supports Marking issues or pull requests as a duplicate however this request is "issues" specific (For PR support another issue can be created).