Make BinderSpawner a real class

As discovered in https://github.com/jupyterhub/binderhub/pull/1010 BinderSpawner is defined in-line in three different places:

• https://github.com/jupyterhub/binderhub/blob/9756642c9b2789695af3edf2ba6802765dbf9531/helm-chart/binderhub/values.yaml#L79
• https://github.com/jupyterhub/binderhub/blob/9756642c9b2789695af3edf2ba6802765dbf9531/testing/minikube/jupyterhub-helm-config.yaml#L23
• https://github.com/jupyterhub/binderhub/blob/9756642c9b2789695af3edf2ba6802765dbf9531/doc/authentication.rst#enabling-authentication

We could reduce duplication in this repo by making it a class, e.g. in binder/spawner.py. As part of this a parameter enable_auth: bool could be added so it’s easier to configure

[betatim]

One thing to consider: this code gets changed (very) rarely. To me this means two things:

• what ever system we use it has to be super simple so we can remember how it works (in two years time maybe none of us will still be working on this!)
• it isn’t worth spending a lot of time on making the process nicer because with the time spent making it nicer we could do several iterations of changes (that XKCD comic)

Maybe we can solve some of the tediousness of the duplication with a comment that points to the duplicated places. We could also put a note in the copy in the docs saying “this is just an example, take these parts we have here and apply them to your version of the spawner/default spawner”.

I’d vote against a process that involves a new package or having to release one of the existing packages/charts (like z2jh). I am not against building our own hub image but I am not at all excited about it. Wary of the tasks that it would introduce like keeping it compatible with the z2jh chart version we use. This seems like a task that would occur frequently compared to the task of copying stuff around when we do change BinderSpawner.

[consideRatio]

Hmmmm, @bitnik an array would be replaced, a dictionary would be merged I think. Hmm… I figure you need to use hub.extraVolumes and hub.extraVolumeMounts directly and potentially conflict with binderhub users that override it.

=) I see that our Jupyter friend has already found this exact issue before, see: https://github.com/helm/helm/issues/3767