Remove unnecessary explicit truth check
Improve generate_user_authentication_include() doc
More generate_user_authentication_include() detail
Replace $REMOTE_USER with $http_x_forwarded_email

After digging deeper into the origin of $REMOTE_USER at Aidan Feldman's
request in 18F/hub#3, I realized that the user's entire email address was
available to nginx's Server Side Include module via the embedded
$http_x_forwarded_email variable, set by ngx_http_core_module:

http://nginx.org/en/docs/http/ngx_http_core_module.html#var_http_

Using this to create each directory under _site/auth seems preferable to
$REMOTE_USER, since having the domain name available as part of the generated
directory name may help to avoid conflicts, should we ever wish to grant
access to users from other domains.
Add "Edit this page" link

Stole the idea from Tom Black:
https://github.com/blacktm/standard-jekyll-template
https://github.com/blacktm/standard-jekyll-template/blob/gh-pages/_config.yml
https://github.com/blacktm/standard-jekyll-template/blob/gh-pages/_layouts/template/main.html
Pull upstream changes into _data/private