When migrating comments, support also updating the body of the original comment (as we do for issues) with a link to the migrated comment.  This way we can also determine when comments have already been migrated.
Just some variable renames that I think improve clarity
Also attempt to fix up cross references when importing comments.
Fix typo in migrated comment message
Added capability to migrate updates to already updated issues (in particular, any new comments that have been added since the migration was first performed, or to migrate the comments from an issue after the issue has been migrated without comments.