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/mail.py

@@ -265,7 +265,7 @@ class MailAccountHandler(LoggingMixin):
         )
 
         correspondent = self.get_correspondent(message, rule)
-        tag = rule.assign_tag
+        tag_ids = [tag.id for tag in rule.assign_tags.all()]
         doc_type = rule.assign_document_type
 
         processed_attachments = 0
@@ -328,7 +328,7 @@ class MailAccountHandler(LoggingMixin):
                     if correspondent
                     else None,
                     override_document_type_id=doc_type.id if doc_type else None,
-                    override_tag_ids=[tag.id] if tag else None,
+                    override_tag_ids=tag_ids,
                     task_name=att.filename[:100],
                 )