Allow setting more than one tag in mail rules

The three migrations do the following to preserve existing data in
assign_tag:
1. Add the new many-to-many field assign_tags.
2. Copy existing data from the assign_tag field to the assign_tags.
3. Delete the existing assign_tag field.

src/paperless_mail/migrations/0009_mailrule_assign_tags.py

@@ -0,0 +1,23 @@
+# Generated by Django 3.2.12 on 2022-03-11 15:00
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("paperless_mail", "0008_auto_20210516_0940"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="mailrule",
+            name="assign_tags",
+            field=models.ManyToManyField(
+                blank=True,
+                related_name="mail_rules_multi",
+                to="documents.Tag",
+                verbose_name="assign this tag",
+            ),
+        ),
+    ]

src/paperless_mail/migrations/0010_auto_20220311_1602.py

@@ -0,0 +1,26 @@
+# Generated by Django 3.2.12 on 2022-03-11 15:02
+
+from django.db import migrations
+
+
+def migrate_tag(apps, schema_editor):
+    # Manual data migration, see
+    # https://docs.djangoproject.com/en/3.2/topics/migrations/#data-migrations
+    #
+    # Copy the assign_tag property to the new assign_tags set if it exists.
+    MailRule = apps.get_model("paperless_mail", "MailRule")
+    for mail_rule in MailRule.objects.all():
+        if mail_rule.assign_tag:
+            mail_rule.assign_tags.add(mail_rule.assign_tag)
+            mail_rule.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("paperless_mail", "0009_mailrule_assign_tags"),
+    ]
+
+    operations = [
+        migrations.RunPython(migrate_tag),
+    ]

src/paperless_mail/migrations/0011_remove_mailrule_assign_tag.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.2.12 on 2022-03-11 15:18
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("paperless_mail", "0010_auto_20220311_1602"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="mailrule",
+            name="assign_tag",
+        ),
+    ]