Replaced disqus with discourse

[travissarles]

I could only partly test this locally because of CORS being disabled. However, it looks like it embeds properly.

[SethTisue]

addresses #744

[heathermiller]

Woohoo! 👍 Ok, merging optimistically – we can revert or add follow-up commits if anything looks amiss.

[SethTisue]

@travissarles this is still on the front page:

on e.g. the tour pages themselves, the Disqus stuff has disappeared, but I don't see any new Discourse stuff yet

[travissarles]

We're having CORS issues. I'm trying to figure it out. Let me know if you have any ideas.

Uncaught DOMException: Failed to read the 'contentDocument' property from 'HTMLIFrameElement': Blocked a frame with origin "http://docs.scala-lang.org" from accessing a cross-origin frame.

VM4658 comments?embed_url=http%3A%2F%2Fdocs.scala-lang.org%2Ftutorials%2Ftour%2F_posts%2F2017-02-13…:16 Failed to execute 'postMessage' on 'DOMWindow': The target origin provided ('https://users.scala-lang.org') does not match the recipient window's origin ('http://docs.scala-lang.org').