Ability to select which conversations they want to merge without having to have the conversations next to one another

We need to be able to merge conversations that aren't in the same list (ie. merge an "open" with an "assigned" or "archived". Also, would be much more efficient to be able to do this from within an open convo (by searching on ID or subject line).

This can be achieved by holding down Ctrl (Windows) or Cmd (Mac) when selecting conversations.