Timestamp comparision with and without timezone information

On mybinder.org we are seeing errors like the following from the notebook. It seems to be harmless (doesn't bother users). Do you know if this has been fixed in 5.7.x or should I make a PR?
```
[E 07:34:16.024 NotebookApp] Exception in callback >
Traceback (most recent call last):
File "/srv/conda/lib/python3.6/site-packages/tornado/ioloop.py", line 1026, in _run
return self.callback()
File "/srv/conda/lib/python3.6/site-packages/notebook/notebookapp.py", line 1579, in shutdown_no_activity
(utcnow() - self.web_app.last_activity()).total_seconds()
File "/srv/conda/lib/python3.6/site-packages/notebook/notebookapp.py", line 372, in last_activity
return max(sources)
TypeError: can't compare offset-naive and offset-aware datetimes
```

We are (still) on notebook 5.6.0.

Start with notebook/notebookapp.py, especially NotebookApp.shutdown_no_activity and last_activity, and trace the timestamp sources shown in the traceback. Reproduce the mixed timezone-aware and timezone-naive comparison on the reported version; done means the callback no longer raises this TypeError.

Written by the indexing model from the issue text.