Extract conversations logic from theme partials.

[andrewculver]

Normally I'd open this issue in the bullet_train-conversations repository, but since it's a private repository I thought it would be better if we had the conversation here. There is a metric ton of conversations business logic in the theme partial views for conversations. I'm normally not fussed about that sort of thing, but in this case I think it's actually worth doing something about. My main concern here is that the conversations logic will end up getting duplicated across a ton of themes that get produced, and it will be hard to keep them all up-to-date and working, etc. I think we need to extract as much common stuff as possible into views, view objects, or even just the models that live in bullet_train-conversations, and as much as possible isolate the views in the themes to be concerned only with styling.