facebook: Update README

Facebook changed the layout of their OAuth2 developer documentation
website. Add new URLs with information relevant to us.

Fixes: https://bugzilla.gnome.org/726609
facebook: Update the code to request a compact web UI

Earlier we were using m.facebook.com as the host in the authorization
URI. That does not work anymore. Now we need to use www.facebook.com
and display=popup.

According to:
https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow
https://developers.facebook.com/docs/reference/dialogs/oauth/

Fixes: https://bugzilla.gnome.org/726609