Upgrade to Django 3.0.0, remove usage of deprecated APIs, upgrade libs

openbook_auth/checkers.py

openbook_auth/models.py

openbook_auth/views/proxy/views.py

openbook_categories/i18n/i18n.py

openbook_categories/models.py

openbook_circles/models.py

openbook_circles/validators.py

openbook_common/checkers.py

openbook_common/i18n/i18n.py

openbook_common/models.py

openbook_common/serializers_fields/request.py

openbook_common/validators.py

openbook_common/views.py

openbook_communities/models.py

openbook_communities/views/community/views.py

openbook_devices/models.py

openbook_devices/validators.py

openbook_hashtags/models.py

openbook_hashtags/validators.py

openbook_importer/views.py