Fix site under HTTPS

The site under HTTPS, https://18f.github.io/doi-extractives-data/ is currently totally broken, with lots of warnings.

This fixes that, mostly by using `site.baseurl` instead of `site.url` wherever possible, and adjusting the URLs to not use trailing slashes for consistency.

I also updated a few other URLs to use `https://`, such as the Open Street Map tile API call.