Reparent comment thread

[cellio]

Prompted by meta:290455

Sometimes an answer gets deleted but there was broader content in the comments that's worth preserving, or a thread attached to a question is really about one of the answers. Can we allow moderators to reparent a comment thread? This could be done via the thread Tools menu. The user should be prompted for a post ID or link (as from Copy Link), which must be for a post (top-level or answer) on that page. (We don't want to enable accidental migration of comment threads to entirely different posts, I don't think.)