refactors tag model to fit ordered collection structure

bookwyrm/migrations/0017_auto_20201128_1849.py

@@ -0,0 +1,42 @@
+# Generated by Django 3.0.7 on 2020-11-28 18:49
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookwyrm', '0016_auto_20201128_1804'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='tag',
+            unique_together=set(),
+        ),
+        migrations.RemoveField(
+            model_name='tag',
+            name='book',
+        ),
+        migrations.RemoveField(
+            model_name='tag',
+            name='user',
+        ),
+        migrations.CreateModel(
+            name='UserTag',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('created_date', models.DateTimeField(auto_now_add=True)),
+                ('updated_date', models.DateTimeField(auto_now=True)),
+                ('remote_id', models.CharField(max_length=255, null=True)),
+                ('book', models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='bookwyrm.Edition')),
+                ('tag', models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='bookwyrm.Tag')),
+                ('user', models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'unique_together': {('user', 'book', 'tag')},
+            },
+        ),
+    ]