Account: add a dashboard url endpoint.

This lets servers share a dashboard url with clients so that they in
turn can present clients with a settings dashboard.
We currently use it on the main server, but self-hosted servers may
also benefit from it for letting users manage some of their settings
(e.g. 2FA).

django_etebase/app_settings.py

@@ -61,6 +61,13 @@ class AppSettings:
             return self.import_from_str(func)
         return None
 
+    @cached_property
+    def DASHBOARD_URL_FUNC(self):  # pylint: disable=invalid-name
+        func = self._setting("DASHBOARD_URL_FUNC", None)
+        if func is not None:
+            return self.import_from_str(func)
+        return None
+
     @cached_property
     def CHUNK_PATH_FUNC(self):  # pylint: disable=invalid-name
         func = self._setting("CHUNK_PATH_FUNC", None)