Wait for Postgres to become available instead of naively sleeping before running Alembic migrations

[jaredtkatz]

Wait for the postgres Docker Compose service to become available instead of naively sleeping for 5 seconds before running Alembic migrations. Implemented using the popular wait-for-it.sh script (added to the repo as a git submodule).

[Buuntu]

I'd rather the wait for it script be copied into the scripts directory than maintaining it as a submodule personally. That script is made to be copy+pasted anyway.