rel="canonical"

This adds a `rel="canonical"` link to the top of every page, with an absolute URL to `https://www.notalone.gov`, so that search engines aren't confused about any instance of this code on other domains (like `18f.github.io/notalone`). Essentially, it prevents penalization for duplicate content and transfers all pagerank to the canonical domain.

To do this, I also added a `permalink` field to every page -- otherwise, the `page.url` field always adds `index.html` to the end of every canonical URL. I added the base canonical URL to `_config.yml` as a `canonical` field.

For more information on `rel="canonical"`, Google's description: https://support.google.com/webmasters/answer/139066?hl=en

It looks like my text editor also deleted a lot of trailing whitespace (I have it configured to do that automatically on save). You can view the diff without that by adding `?w=1` to the end of this pull request's URL.