See #170: cover on tracks and artists

...unkwhale_api/common/migrations/0007_auto_20200116_1610.py

api/funkwhale_api/common/models.py

api/funkwhale_api/common/mutations.py

api/funkwhale_api/common/utils.py

api/funkwhale_api/federation/serializers.py

api/funkwhale_api/federation/views.py

api/funkwhale_api/manage/serializers.py

api/funkwhale_api/manage/views.py

api/funkwhale_api/music/factories.py

api/funkwhale_api/music/metadata.py

...funkwhale_api/music/migrations/0047_auto_20200116_1246.py

api/funkwhale_api/music/models.py

api/funkwhale_api/music/mutations.py

api/funkwhale_api/music/serializers.py

api/funkwhale_api/music/tasks.py

api/funkwhale_api/music/views.py

api/requirements/test.txt

api/tests/common/test_models.py

api/tests/common/test_mutations.py

api/tests/common/test_utils.py