Finalize new URL

Finishes the process #216 started, to update the site to point to the new URL.

* Updated `_config.yml` to change the site URL to be `https://project-open-data.cio.gov`
* Adds a `rel=canonical` link tag to point search engines et al to `https://project-open-data.cio.gov/`
* Any links that pointed to the old URL have now been changed to relative paths

A repo administrator still needs to update the URL visible at the [top of the repo page](https://github.com/project-open-data/project-open-data.github.io) to point to `https://project-open-data.cio.gov/`.