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.

- Use the singular (you can only select one from each list; you can't select
  "workflows");
- Simplifying wording (nobody needs to see “ESFRI” here!).

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.