Implement tag models

api/config/settings/common.py

...unkwhale_api/common/management/commands/load_test_data.py

api/funkwhale_api/common/migrations/0003_cit_extension.py

api/funkwhale_api/factories.py

api/funkwhale_api/music/factories.py

api/funkwhale_api/music/filters.py

api/funkwhale_api/music/migrations/0004_track_tags.py

...funkwhale_api/music/migrations/0027_auto_20180515_1808.py

api/funkwhale_api/music/models.py

api/funkwhale_api/music/serializers.py

api/funkwhale_api/music/views.py

api/funkwhale_api/radios/radios.py

api/funkwhale_api/tags/__init__.py

api/funkwhale_api/tags/admin.py

api/funkwhale_api/tags/factories.py

api/funkwhale_api/tags/migrations/0001_initial.py

api/funkwhale_api/tags/migrations/__init__.py

api/funkwhale_api/tags/models.py

api/funkwhale_api/users/factories.py

api/funkwhale_api/users/models.py