Upgrade Django to current LTS version

Upgrade Django to the current LTS version and the required packages up to the latest minor versions

• Django==2.2.12
• django-cors-headers==3.2.1
• django-oidc-provider==0.7.0
• django-tinymce==2.8.0
• django-versatileimagefield==1.11
• djangorestframework==3.11.0
• drf-nested-routers==0.91
• Pillow==4.3.0
• python-dateutil==2.8.1
• PyYAML==3.13

Close #31 (closed)