Discussion forum

Hey guys!

I was investigating some possible options for hosted forums etc, and the more time I spent doing that, the more I realised that github issues itself really isn't far off from a discussion forum anyway.

So I propose the following: For discussions, let's just use the newly-created `discussion` label here. As far as when to close a discussion issue it's a bit ambiguous, but let's try to tidy up after ourselves.

On the one hand, issues is meant to be a bug tracker, but I think it could work well for discussions as well, as we've already been using it.

The other nice thing is that with the label you essentially have a [forum permalink](https://github.com/inkle/ink/labels/discussion) too.