rel="canonical"

Re-implementing #4 after merge conflicts. Reproducing original PR message:

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.