Feature idea/request: Deduplication of issues

[basicallydan]

In a real-life retrospective with post-it notes, it's easy to group together issues by moving them around in meatspace.

But on Slack it's hard! You gotta copy + paste and all this crazy keyboard stuff. What if we could use emoji reactions to say which things are the same, then say @retrobot dedupe and it gives a new, depued list for voting.