Add debug log for static file paths

[jasongrout]

Too often I'm trying to track down an errant extra static file path (like some javascript installed in some obscure place). This makes it easy to see exactly what is being served.

[jasongrout]

CC @dmadeka

[jasongrout]

Perhaps log() doesn't have to be a function? I'm not sure what the specific lifetime concerns are with the application initialization check. @minrk - would the following work just as well?

    if Application.initialized():
        log = Application.instance().log
    else:
        log = app_log

at the top level?

[takluyver]

@jasongrout FileFindHandler, which you're modifying, inherits from IPythonHandler, which defines the log property. So you should be able to use self.log.debug(...), I think, without needing an extra function.

The application won't be initialised at import time, so putting that check at the top level of the module wouldn't work.

[minrk]

Arg, I wrote a whole comment last week, but apparently GitHub lost it. What @takluyver said - module-level initialization won't work because import happens too early. This function is fine as-is. @takluyver it's needed here because this method is required to be a classmethod.

[gnestor]

@jasongrout Does this need to be backported before releasing 4.3?

[minrk]

Backported