At the moment, this is unused so it makes no practical difference. Longer
term, however, we shouldn't hard-code a value here, but rather handle it in
the same way as workflows and IDA systems.

These services are not all JupyterHub systems. Indeed, at time of writing the
only service we've actually configured in esap-api-gateway is MyBinder, which
isn't a JupyterHub at all. Calling them “jhub” or similar is going to cause
confusion.

Apart from it just being ugly to hard-code these URLs in the source, they can
be wrong & misleading when the default doesn't actually correspond to a
service which is configured in esap-api-gateway.

(Jira ticket SDC-230)

moved ui_schema into a function, may needs to render different ui_schema for different catalog. also fixed some front-end bug, e.g. SDC-97