This PR adds the necessary resolvers to support
the graphQL front-end. Some additional methods
need to be added to the model structs to support
this. Comments have been made to clarify their purpose.
There is a slight duplication of code in the api layer.
But eventually that's going to go away when we move
away from the REST API entirely.
The schema is in api4/schema.graphqls and gets compiled
into the binary as an asset.
The GraphiQL editor url is at GET /api/v5/graphql.
Everything is behind the feature flag MM_FEATUREFLAGS_GRAPHQL.
```release-note
NONE
```
