Put all config in a k8s Secret, don't split it up into ConfigMap/Secret

[consideRatio]

We currently have a configmap.yaml and secret.yaml, and split up information from .Values into them based on if the content is sensitive or not. This is a cumbersome practice that adds complexity which is prone to break - this is based on experience of it breaking :D In z2jh we decided to merge them.

I added a note in the configmap saying....

  FIXME: Stop splitting up parts into configmap/secret as it adds complexity,
         instead only use a secret. For the equivalent PR in z2jh, see:
         https://github.com/jupyterhub/zero-to-jupyterhub-k8s/pull/1682

jupyterhub/zero-to-jupyterhub-k8s#1682 can provide an example on how we went about it in z2jh, and I suggest that we mimic this here.