Replace local secure_write method with jupyter core's

[kevin-bates]

When the secure_write machinations were taking place, we added an implementation to jupyter_server before it found its final resting place in jupyter_core. The second half of this notebook issue comment exposed that its replacement was never followed up. We'll track its replacement via this issue.

[Zsailer]

Ah, yes. This was started in #357, but closed later.

[kevin-bates]

Ok. Thanks for the pointer. This lead to using core's versions of is_hidden, is_file_hidden, and exists as well. PR on its way.