Resolve "Adding cover art to my albums"

api/funkwhale_api/common/factories.py

...unkwhale_api/common/migrations/0005_auto_20191125_1421.py

api/funkwhale_api/common/models.py

api/funkwhale_api/common/serializers.py

api/funkwhale_api/common/views.py

api/funkwhale_api/federation/serializers.py

api/funkwhale_api/music/mutations.py

api/tests/common/test_tasks.py

api/tests/conftest.py

api/tests/federation/test_routes.py

api/tests/federation/test_serializers.py

api/tests/music/test_mutations.py

changes/changelog.d/588.enhancement

front/src/components/common/AttachmentInput.vue

front/src/components/library/EditCard.vue

front/src/components/library/EditForm.vue

front/src/edits.js