Making disqus installation more robust

[nestorlafon]

Hello

With this PR I'm trying to make Disqus work independently if of your domain URL, in case you don't know it or have to change it.

I had some problems getting Disqus to work on my new site. After some trial error, I found that the problem was that this.page.url was not an absolute_url as Disqus needs (https://help.disqus.com/en/articles/1717084-javascript-configuration-variables).

Adding url:<you prod base URL> to the Jekyll configuration (_config.yml) solved the problem without changing the code, but, as for me, some people might want not to set it.

Also, in the Disqus documentation, they recommend using a this.page.identifier that doesn't change if the domain change, to retain the comments in that case. I changed that to be the relative URL (page.url). I was tempted to use page.id but non-post pages don't have one and the page.id is page.url without the file extension

There is an open PR #536 that could be closed with this one and that shows figuring out the Disqus error is not trivial for normal users.

I'm neither a JS nor Jekyll expert. Any comment or ideas making this code better are welcome! 😃