nbextension install output should go to stdout

The refactor in 4.2 switched to using the logging module for nbextension install, but this goes to stderr by default. We shouldn't write anything to stderr when installing or uninstalling an extension without an error.

In general, I think using the application logger is probably not the right thing to do for most of the output in nbextension/serverextension actions, and we should be using print to stdout, stderr instead.

Start by tracing the nbextension install and uninstall actions, then compare how nbextension/serverextension actions use the application logger. Done means successful installation and uninstallation output goes to stdout, while stderr is reserved for errors; the broader output behavior should be resolved consistently across these actions.

Written by the indexing model from the issue text.