Add Disqus support

[choldgraf]

Disqus is another popular provider for online commenting platform. It seems like we should be able to support Disqus boxes as well:

• Disqus embed instructions

  • https://help.disqus.com/en/articles/1717112-universal-embed-code
  • https://help.disqus.com/en/articles/1717084-javascript-configuration-variables

• There is also this Sphinx extension for Disqus which could be a good starting point: https://github.com/Robpol86/sphinxcontrib-disqus

• An ablog also has a much simpler Disqus integration which could be easier to implement: https://github.com/sunpy/ablog/blob/d148cf469d3ffa1fcac48c3032ea5192b4e19ae0/ablog/templates/page.html#L29