Ensure Base URL configuration is working

[echarles]

base_url configuration result in 404

 jupyter server --ServerApp.jpserver_extensions="{'simple_ext1': True}" --ServerApp.base_url=/mybase
... snip...
To access the server, open this file in a browser:
  http://127.0.0.1:8888/mybase/mybase/?token=5e499185d1197ed235ed20133016e6a86a34f46eef425802

Please note the /mybase/mybase/ doubled base url.

It shows 404 for all following urls:

• http://127.0.0.1:8888/mybase/mybase/?token=5e499185d1197ed235ed20133016e6a86a34f46eef425802
• http://127.0.0.1:8888/mybase/?token=5e499185d1197ed235ed20133016e6a86a34f46eef425802
• http://127.0.0.1:8888/?token=5e499185d1197ed235ed20133016e6a86a34f46eef425802

[Zsailer]

Fixed in #280.