workshops/migrations/0021_auto_20141214_2232.py: Add data migration

This name consolidation is almost exactly the example given in the
Django docs [1].  I haven't separated the schema migration from the
data migration, because it's a simple enough migration and splitting
it into three (add Person.name, populate it, and remove the other name
fields) seemed like overkill.

This migration condenses our earlier personal-middle-family ordering
to a single point in time.  Users who perfer other oderings will have
their names mangled by the data migration, but they can fix the
ordering after the migration and then enjoy the correct ordering ;).

[1]: https://docs.djangoproject.com/en/1.7/topics/migrations/#data-migrations