Provide a way of changing the default service prefix /services/nbexchange/ for plugins

[ykazakov]

According to README, the extension point on which the nbexchange service is running can be configured by setting the environment variable JUPYTERHUB_SERVICE_PREFIX. (Is it intended for use with JupyterHub?)

However, for nbgrader plugins, this path seems to be hardcoded:

nbexchange/nbexchange/plugin/exchange.py

Lines 33 to 36 in 985796a

	def service_url(self):
	this_url = urljoin(self.base_service_url, "/services/nbexchange/")
	self.log.debug(f"service_url: {this_url}")
	return this_url

Please provide a convenient way of changing this value. Preferably as a configuration option that can be set in nbgrader_config.py similarly as for setting up the exchange.