Added application token for easier auth

api/config/settings/common.py

api/funkwhale_api/common/authentication.py

api/funkwhale_api/users/factories.py

api/funkwhale_api/users/migrations/0020_application_token.py

api/funkwhale_api/users/models.py

api/funkwhale_api/users/oauth/serializers.py

api/funkwhale_api/users/oauth/views.py

api/tests/common/test_authentication.py

api/tests/users/oauth/test_views.py