Rename creds key email -> public_email

Partial revert of 64b65ca where this was broken, causing a name
collision in the creds extra association list.

Yesod/Auth/OAuth2/Github.hs

@@ -107,7 +107,7 @@ toCreds user userMails token = Creds
         , ("access_token", decodeUtf8 $ accessToken token)
         ]
         ++ maybeExtra "name" (githubUserName user)
-        ++ maybeExtra "email" (githubUserPublicEmail user)
+        ++ maybeExtra "public_email" (githubUserPublicEmail user)
         ++ maybeExtra "location" (githubUserLocation user)
     }