Set app log level to logging.DEBUG instead of logging.INFO

Recently I tried using logging to record debugging information in my callbacks via app.logger (see discussion at https://community.plot.ly/t/logging-debug-messages-suppressed-in-callbacks) and found that some debug messages were being ignored, apparently only when they were logged from callbacks.

It turned out that with the default behaviour (i.e. silence_routes_logging=True), debug messages don't get logged after the server is started as Dash sets self.logger.setLevel(logging.INFO) when app.run_server() is called (see https://github.com/plotly/dash/blob/aac493c23366043c3e9612c3f5624ffacbb5d1e9/dash/dash.py#L1162). It's easy to work around, but @ned2 suggested that it might be more sensible for Dash to set its master log level to logging.DEBUG anyway. As an end-user it was not obvious to me why my debug messages were being logged at first and then weren't.

If this would be a sensible change, I'm happy to make a mini-PR.

Start in dash/dash.py at the app.run_server() path referenced by the issue, especially where the logger level is set. Check how silence_routes_logging affects callback messages and identify the relevant logging behavior to verify. Done means debug messages from callbacks remain available after the server starts without requiring an end-user workaround.