Links should be able to connect more than one word; more than one link

[angusforbes]

Right now, links only connect one word to another word. But it makes sense that a link could represent a relationship with multiple words (or multiple links, or both). Should be straightforward to add. But will need to replace the "startWord" / "endWord" with a sorted array. Words will never change position, but links might -- need to think about how to handle interactions where link attachments to other links could change order.