Pass through real client IP address for JupyterHub

[rkevin-arch]

When testing something on Galaxy I saw stuff like:

[I 2020-11-28 09:39:18.176 JupyterHub log:174] 200 GET /hub/spawn (rk@rkevin.dev@10.0.0.118) 29.21ms

It thinks I logged in from the IP address 10.0.0.118, which happens to be star4, where the nginx ingress is running. Ideally, we want to expose the client's IP address for logging purposes (and maybe to blacklist certain IP ranges in the future if we see abuse of our servies).

Apparantly this is an issue on Flock as well, even if we aren't using an ingress (but that's because it runs autohttps (traefik) to deal with https traffic, which acts like an ingress anyway):

[I 2020-11-28 09:48:12.364 JupyterHub log:174] 200 GET /hub/spawn (krong@ucdavis.edu@10.0.0.110) 21.29ms

See also: kubernetes/ingress-nginx#6163