authenticate the static handler

* Closes #231
* Provide our own static handler rather than using Jupyter Server.
* This is because we are not using the same template/static resource approach.
* We would have to edit URLs in the UI to point at `static/cylc/*`.
* Adding authentication here provides login redirection.

cylc/uiserver/handlers.py

@@ -151,7 +151,12 @@ def hub_groups(self):
 
 
 class CylcStaticHandler(CylcAppHandler, web.StaticFileHandler):
-    pass
+
+    @web.authenticated
+    def get(self, path):
+        # authenticate the static handler
+        # this provides us with login redirection and token cashing
+        return web.StaticFileHandler.get(self, path)
 
 
 class UserProfileHandler(CylcAppHandler):