Change initdb to only use migration on existing databases.
Change initdb to only use migration on existing databases.

Before running `make` in local development would stamp an existing DB
with the lastest version first, and then try to run any missing migration
steps. Now we check if there's an existing alembic_version first, and if
so only run the migration steps.

mozilla/ichnaea,therewillbecode/ichnaea,mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea,therewillbecode/ichnaea,therewillbecode/ichnaea