DEV: Add IPython.html to the default template path.

This makes it possible to override a file with a template that also
inherits from that file.

For example, this makes it possible to override a single block of
notebook.html by creating a file called notebook.html that inherits from
templates/notebook.html.

IPython/html/__init__.py

@@ -3,8 +3,22 b''
 import os
 # Packagers: modify this line if you store the notebook static files elsewhere
 DEFAULT_STATIC_FILES_PATH = os.path.join(os.path.dirname(__file__), "static")
-# Packagers: modify this line if you store the notebook template files elsewhere
-DEFAULT_TEMPLATE_FILES_PATH = os.path.join(os.path.dirname(__file__), "templates")
+
+# Packagers: modify the next line if you store the notebook template files
+# elsewhere
+
+# Include both IPython/html/ and IPython/html/templates/.  This makes it
+# possible for users to override a template with a file that inherits from that
+# template.
+#
+# For example, if you want to override a specific block of notebook.html, you
+# can create a file called notebook.html that inherits from from
+# templates/notebook.html, and the latter will resolve correctly to the base
+# implementation.
+DEFAULT_TEMPLATE_PATH_LIST = [
+    os.path.dirname(__file__),
+    os.path.join(os.path.dirname(__file__), "templates"),
+]
 
 del os
 

IPython/html/notebookapp.py

@@ -53,7 +53,7 b' from tornado.log import LogFormatter, app_log, access_log, gen_log'
 
 from IPython.html import (
     DEFAULT_STATIC_FILES_PATH,
-    DEFAULT_TEMPLATE_FILES_PATH,
+    DEFAULT_TEMPLATE_PATH_LIST,
 )
 from .base.handlers import Template404
 from .log import log_request
@@ -536,8 +536,8 b' class NotebookApp(BaseIPythonApplication):'
 
     @property
     def template_file_path(self):
-        """return extra paths + the default location"""
-        return self.extra_template_paths + [DEFAULT_TEMPLATE_FILES_PATH]
+        """return extra paths + the default locations"""
+        return self.extra_template_paths + DEFAULT_TEMPLATE_PATH_LIST
 
     nbextensions_path = List(Unicode, config=True,
         help="""paths for Javascript extensions. By default, this is just IPYTHONDIR/nbextensions"""