make server_extension_path an overrideable property

matches nbextension_path, etc. from NotebookApp

This should be overridden when disabling user config in JupyterHub

jupyter_server/serverapp.py

@@ -2139,6 +2139,15 @@ def init_components(self):
         # TODO: this should still check, but now we use bower, not git submodule
         pass
 
+    @property
+    def server_extension_path(self):
+        """Path to search for server extension config"""
+        config_path = jupyter_config_path()
+        if self.config_dir not in config_path:
+            # add self.config_dir to the front, if set manually
+            config_path.insert(0, self.config_dir)
+        return config_path
+
     def find_server_extensions(self):
         """
         Searches Jupyter paths for jpserver_extensions.
@@ -2154,11 +2163,7 @@ def find_server_extensions(self):
         # This enables merging on keys, which we want for extension enabling.
         # Regular config loading only merges at the class level,
         # so each level clobbers the previous.
-        config_paths = jupyter_config_path()
-        if self.config_dir not in config_paths:
-            # add self.config_dir to the front, if set manually
-            config_paths.insert(0, self.config_dir)
-        manager = ExtensionConfigManager(read_config_path=config_paths)
+        manager = ExtensionConfigManager(read_config_path=self.server_extension_path)
         extensions = manager.get_jpserver_extensions()
 
         for modulename, enabled in sorted(extensions.items()):