Upgrade requirements up to the latest major versions

- Django==1.11.29
- 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

Later, upgrade
- Django up to 2.2.11, the current LTS version
- django-versatileimagefield up to 2.0, the current stable version
- Pillow up to 7.0.0, the current stable version
- PyYAML up to 5.3.1, the current stable version