Add a warning to prevent users from copy/pasting an entire board's worth of content

We’re working on a shared board with dozens of folks creating hundreds of sticky notes. If one single user accidentally highlights a large number of objects and then unwittingly selects CTRL+C/CTRL+V, we’ve had a few instances of the entire board duplicating over everyone’s work.

A quick fix would be to pop up a dialog box warning users that they’re about to paste a large number of objects and require a quick manual confirmation. This dialog could also have a “stop popping this dialog up for a little bit” option for folks working on moving large amounts of information around large boards.