add disqus to footer on each page

[lheagy]

this adds a comment thread to the bottom of each page of em geosci using disqus (https://disqus.com/) to allow posting of comments directly in the site

• privacy policy: https://help.disqus.com/customer/portal/articles/466259-privacy-policy

@dougoldenburg : I would appreciate your thoughts on this

For anyone interested in helping moderate / respond to comments, please let me know and I will add you on the disqus team!