Add conversation OPs

If someone sets a new conversation that hasn't been used on that board before, they'll be the OP of it, if other people set the same convo and you hover over it it'll show the OP post.

Convo OPs are stored in a separate collection as well as the normal posts collection, so they'll last longer than normal posts which have a ~100 post limit.

Right now there's nothing to purge convos with, maybe we could add something to purge it if it's had no replies in x days