In the `mergestat` chart, handle starting `graphql` after the `worker` (migrations) are completed

[patrickdevivo]

We have an annoying race condition. Sometimes the graphql deployment starts up before the worker has finished running DB migrations. This means the GraphQL API Graphile serves up is out of date from the actual schema of the DB, often producing errors in the console/frontend.

In the docker-compose setup, we're able to handle this constraint. We should implement a similar solution (not sure how) in our helm chart to avoid this situation here.