Backport PR #4035: Fixed custom jinja2 templates being ignored when setting template_path

IPython/html/notebookapp.py was ignoring user specified templates by setting the jinja2 environment to the default template directory before updating template_path.  This fix allows a user defined template_path to propagate to the jinja2 environment.

IPython/html/notebookapp.py

@@ -149,7 +149,7 b' class NotebookWebApplication(web.Application):'
         # Note that the URLs these patterns check against are escaped,
         # and thus guaranteed to be ASCII: 'héllo' is really 'h%C3%A9llo'.
         base_project_url = py3compat.unicode_to_str(base_project_url, 'ascii')
-        template_path = os.path.join(os.path.dirname(__file__), "templates")
+        template_path = settings_overrides.get("template_path", os.path.join(os.path.dirname(__file__), "templates"))
         settings = dict(
             # basics
             base_project_url=base_project_url,