Revert "Move django-extensions to dev requirements."

This reverts commit 84a5e23f.

requirements-dev.txt

 -r requirements.txt
 
 django-debug-toolbar==2.2
-django-extensions==2.2.9
 factory_boy==2.12
 flake8==3.8.2
 isort==4.3.21

requirements.txt

@@ -10,6 +10,7 @@ django-anymail==7.1.0
 django-basic-auth-ip-whitelist==0.3.3
 django-bleach==0.6.1
 django-countries==6.1.2
+django-extensions==2.2.9
 django-filter==2.2.0
 django-fsm==2.7.0
 django-heroku==0.3.1