Add config option to set the site width to full width

[Scott Weber]

https://federalist-report-template.18f.gov/ has a 100% site width feel.
Add this as an option to the config.yml file

[Aidan Feldman]

For https://federalist-proxy.app.cloud.gov/site/18f/uswds-jekyll/docs/ specifically, right?

[Scott Weber]

This should be a site wide change meaning it would apply to any layout/page on the site