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/admin.py

@@ -82,7 +82,7 @@ class MailRuleAdmin(admin.ModelAdmin):
                 ),
                 "fields": (
                     "assign_title_from",
-                    "assign_tag",
+                    "assign_tags",
                     "assign_document_type",
                     "assign_correspondent_from",
                     "assign_correspondent",