Feature: Merge votes for duplicate issues

[Squelch]

We have been using this plugin for some time now. With a very active tracker, it is possible to overlook duplicate issues. If during the overlap, several people have voted for this latest issue, the original issue loses out on those votes when the new one is set to duplicate.

The idea that appears to be the most workable, is the adding of all of the unique votes in the duplicate to the original when the status or relation is set.

We will attempt to implement this ourselves, but any further suggestions or warnings of pitfalls would be gladly received by inexperienced Ruby programmers.