Updates Github link to go directly to the edit page

This updates the "Edit on GitHub" link for the sphinx rtd theme to go directly to the editing screen for that page. 

Github now handles the case of a logged-out user correctly, presenting them with a login screen that redirects back to the same URL after logging in or signing up. Previously, Github would 404 in this case.

/cc @jamesturk, since I was inspired to file this by the Edit on GitHub link at the top of http://docs.opencivicdata.org/en/latest/