Add boilerplate for Google sign-in

For more information, read:
https://developers.google.com/identity/sign-in/web/sign-in?authuser=1

TODO: Use Elm ports or something similar to properly interop with the onSignIn
and signOn functions defined in index.html.

client/src/State.elm

@@ -22,7 +22,7 @@ type alias Model =
 init : Model
 init =
     { isLoading = False
-    , view = Landing
+    , view = Login
     }