remove stopped users from proxy on startup

We already added running users, but we didn't handle removing users from the proxy
if the user's server was stopped (e.g. while the Hub was restarting).

jupyterhub/user.py

@@ -161,9 +161,9 @@ class User(HasTraits):
     @property
     def proxy_path(self):
         if self.settings.get('subdomain_host'):
-            return url_path_join('/' + self.domain, self.server.base_url)
+            return url_path_join('/' + self.domain, self.base_url)
         else:
-            return self.server.base_url
+            return self.base_url
     
     @property
     def domain(self):