Ability to move comments from one discussion to another

[srallen]

We've had a request on Talk for a feature to be able to move comments from one discussion to another. There currently does not exist in the UI an ability for mods to do this. Additionally, it's unclear since the API is largely undocumented if there's an API route for this, but as far as the current dev team knows, there is not.

The comments model appears to have an update method: https://github.com/zooniverse/Talk-Api/blob/68bf124633a70f9084258589ff690f43ad3d2a48/app/models/comment.rb#L111-L117

but this doesn't seem to be what we need. There's an update board method that is closer to what is need, but the request is for updating the discussion id:

https://github.com/zooniverse/Talk-Api/blob/68bf124633a70f9084258589ff690f43ad3d2a48/app/models/comment.rb#L119-L121

Similarly, although the original request is about moving existing comments from one existing discussion to another existing discussion, some project builders wish for a true merge functionality which would be two discussion being merged to create a new discussion with the comments from the two previous.