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/