Fix #108 - Add data migration to import the Crowdtilt keys from .env

If a settings object exists and has the ct_production_guest_id and
ct_production_admin_id attributes, the site has already been intialized
for payments with Crowdtilt, so we import the CROWDTILT_PRODUCTION_KEY and
CROWDTILT_PRODUCTION_SECRET vars from .env and update the settings
object with them.