Update django-constance to 2.4.0

constance/__init__.py

constance/admin.py

...e/backends/database/migrations/0002_auto_20190129_2304.py

constance/backends/database/models.py

constance/base.py

constance/changes.rst

constance/checks.py

constance/static/admin/css/constance.css

constance/static/admin/js/constance.js

constance/templates/admin/constance/change_list.html

...ance/templates/admin/constance/includes/results_list.html