move_issue command should also move any existing comments

[carbonin]

If some conversation took place in the original issue before it was discovered that it needed to be moved, those comments would be stuck in the old issue. It would be better to have all the context in the same place. This would also presumably carry over mentions and add mentions to people who might have commented, but were not the reporter.

Here is an issue where I had to manually copy over content from a comment I made to the new issue after a move: ManageIQ/manageiq#20172

[Fryguy]

Similar to #347. I actually think the best way to handle comments is to not do our own transfer, but instead leverage the built-in GitHub transfer. Built-in doesn't handles labels, but it does handle comments. Only concerns are that it might require higher-level access than just write, and I'm not sure there's an API available for issue transfer.