Create a migration script for new fernet encrypted keys.

[kevgliss]

With #123 in place, we will need a script to be able to move from the older style sqlalchemy_utils encrypted column to the new fernet encrypted column.

This will be handled via a alembic migration that will create an intermediate table such that we can convert between the new encryption scheme and the old scheme. New deployments should not have to undergo this migration.

[kevgliss]

Closed by #128