Start PEDSnet development

This commit breaks from previous development, which was toward a
demo-able app built on the OSIM2 simulated dataset. The new development
path is towards a production-capable app built on the OMOP CDMV4 data
model for the PEDSnet Data Coordinating Center. In that spirit, this
commit removes all migrations and starts over with new models, beginning
with person, provider, care_site, organization, location, visit,
condition, occurrence, and drug. These models are intended to sit on top
of materialized views, the code for which is yet to come.

Signed-off-by: Aaron Browne <aaron0browne@gmail.com>