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.
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.
