Only handle .ipynb files

[jtpio]

For now the VoilaHandler can only process notebook files (.ipynb).

However the handler is currently using path_regex from jupyter_server.base.handlers, which catches all files:

https://github.com/QuantStack/voila/blob/cadc6e801a0a4537ee216fdfe0b67682e054d9ba/voila/app.py#L37

As a result, some non .ipynb files can hit the VoilaHandler:

WARNING:tornado.general:404 GET /voila/render/b06871f281fee6b241d60582ae9369b9.ttf (127.0.0.1): No such file or directory: b06871f281fee6b241d60582ae9369b9.ttf

It would make sense to use a custom path_regex to only catch .ipynb files (at least for now).