The documents.tests.test_migration_mime_type test suite failes if no
backwards migration is provided. This simple backwards migration sets
the old assign_tag field with a tag if exactly one is set in
assign_tags.
This fixes the errors in the tests that the new many-to-many
assign_tags property cannot be accessed due to MailRule not having an
id. In one case it it necessary to give additional names to both objects
as several are created.
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.
jonasc