Use pre_spawn and post_stop hooks instead of polling for setting up publish service

[vpavlin]

Currently there is a simple service which polls proxy API and updates proxy for published notebooks route. This could be done without polling by using pre_spawn and post_stop hooks in the spawner which would only configure the new proxy route once

i.e. remove https://github.com/AICoE/jupyterhub-ocp-oauth/blob/master/.jupyter/jupyterhub_config.py#L24

add the public route setup/teardown to https://github.com/AICoE/jupyterhub-ocp-oauth/blob/master/.jupyter/jupyterhub_config.py#L303