Display deprecation warnings in dev mode
On our development workstations, when we run the application, we really want to always see the deprecation warnings.
Django upstream used to show them by default, but they changed that with 1.11.
We've at least missed on !833 (merged) because of this, which we discovered rather by chance. There could be others.
We need to try and restore that to make it easier to spot problems in our code and fix them, but only on our development workstations, not on production servers.
Edited by Mathieu Bridon