add method to enable extensionapps to work with classic notebook server

Author: Zsailer

docs/source/developers/extensions.rst

@@ -299,6 +299,23 @@ To make your extension executable from anywhere on your system, point an entry-p
         }
     )
 
+``ExtensionApp`` as a classic Notebook server extension
+-------------------------------------------------------
+
+An extension that extends ``ExtensionApp`` should still work with the old Tornado server from the classic Jupyter Notebook. The ``ExtensionApp`` class 
+provides a method, ``load_classic_server_extension``, that handles the extension initialization. Simply  define a ``load_jupyter_server_extension`` reference
+pointing at the ``load_classic_server_extension`` method: 
+
+.. code-block:: python
+
+    # This is typically defined in the root `__init__.py` 
+    # file of the extension package.
+    load_jupyter_server_extension = MyExtensionApp.load_classic_server_extension
+
+
+If the extension is enabled, the extension will be loaded when the server starts.
+
+
 Distributing a server extension
 ===============================
 

jupyter_server/extension/application.py

@@ -13,6 +13,7 @@
 )
 from traitlets.config import Config
 from tornado.log import LogFormatter
+from tornado.web import RedirectHandler
 
 from jupyter_core.application import JupyterApp
 
@@ -21,7 +22,6 @@
 from jupyter_server.utils import url_path_join
 from .handler import ExtensionHandlerMixin
 
-
 # -----------------------------------------------------------------------------
 # Util functions and classes.
 # -----------------------------------------------------------------------------
@@ -422,6 +422,36 @@ def _load_jupyter_server_extension(cls, serverapp):
         extension.initialize()
         return extension
 
+    @classmethod
+    def load_classic_server_extension(cls, serverapp):
+        """Enables extension to be loaded as classic Notebook (jupyter/notebook) extension.
+        """
+        extension = cls()
+        extension.serverapp = serverapp
+        extension.load_config_file()
+        extension.update_config(serverapp.config)
+        extension.parse_command_line(serverapp.extra_args)
+        # Add redirects to get favicons from old locations in the classic notebook server
+        extension.handlers.extend([
+            (r"/static/favicons/favicon.ico", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/favicon.ico")}),
+            (r"/static/favicons/favicon-busy-1.ico", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/favicon-busy-1.ico")}),
+            (r"/static/favicons/favicon-busy-2.ico", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/favicon-busy-2.ico")}),
+            (r"/static/favicons/favicon-busy-3.ico", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/favicon-busy-3.ico")}),
+            (r"/static/favicons/favicon-file.ico", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/favicon-file.ico")}),
+            (r"/static/favicons/favicon-notebook.ico", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/favicon-notebook.ico")}),
+            (r"/static/favicons/favicon-terminal.ico", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/favicon-terminal.ico")}),
+            (r"/static/logo/logo.png", RedirectHandler,
+                {"url": url_path_join(serverapp.base_url, "static/base/images/logo.png")}),
+        ])
+        extension.initialize()
+
     @classmethod
     def launch_instance(cls, argv=None, **kwargs):
         """Launch the extension like an application. Initializes+configs a stock server