Copy votes when moving an issue or marking duplicates

Description

• Copy upvotes and downvotes, in a best effort way, when moving an issue and marking an issue as a duplicate.
• When moving issue A to issue B, issue A is closed behind the scenes and issue B is created. In a best effort way, copy the upvotes and downvotes to issue B. It is best effort because if of permission issues, a person who upvoted/downvoted in issue A, cannot do so in issue B, that upvote and downvote will not be copied.
• When marking issue A as a duplicate of issue B, issue A is closed. In a best effort way, copy the upvotes and downvotes to issue B. If a person who up/downvoted in issue A cannot do so in issue B (because of permissions), then the vote will not be copied. If that person already voted in B, then do not double count.

Original description

We often get duplicate issues and often close one with a comment pointing to the other. We can copy over any changes necessary to the description, but subscribers, and more importantly, upvotes get lost, which can underrepresent how important an issue. We should have an ability to explicitly merge an issue into another and copy over at least the upvotes.