Include the "Solved" plugin

@blake, when you get the time, could you look at adding the Solved plugin to our Discourse?

I haven’t looked at it in detail, but it would be nice if it could be limited to relevant categories; obviously it would be useful for the Q&A category, probably Feedback as well. It’d make the site a little more Stack-exchange-ish.

Better late than never, right?

So this plugin is now installed. I enabled it for the Q&A category. Feel free to enable it for additional categories if you like.