nbextension install output should go to stdout

[minrk]

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.

[takluyver]

It's fine for application code (e.g. InstallNBExtensionApp) to use simple print(). But for API functions like install_nbextension(), I think it's better for them to use the logging machinery unless writing output to the terminal is explicitly what the function is for - which it's not in this case.

I'm not sure why it matters that stuff is written to stderr instead of stdout, though? I think of stderr as a side-channel to display info to the user when the main output channel is redirected; it doesn't necessarily indicate that something has gone wrong.

[minrk]

It's a minor annoyance to me because I think we are managing output wrong, but no need to block a release for it.