Create data migration for first step of schema transition

Note that this depends on [PR 2](https://github.com/cbmi/varify-data-warehouse/pull/2).  The attached PR shall be invalid until this merge is complete.

This shall be a reversible data migration to complete the first step of the greater transition to a new schema.
What is the purpose of this line? 3 lines down, you set this same this to `None`. I'm guessing this is a duplicate, remove it and the blank line below it.