This change adds the title and body fields to the Changeset in the GraphQL API.
It does this by adding an external_service_type column to changesets and populating it with the value of the column of the same name from repo. We decided that duplicating this data is more economic than using a JOIN on every SELECT.
The marshaling/unmarshaling of the Metadata uses the same technique that we previously used in repo-updater: when we scan a changesets row returned from the database, we look at the external_service_type column to determine in which codehost specific struct we should unmarshal the metadata jsonb column. Right now we only support GitHub and in case external_service_type == "github" we turn changeset.Metadata into a *github.PullRequest.
The changesetResolver then has to do a typeswitch on Metadata to determine how to get to fields like Title and Body.
