Support unsubscribing from GraphQL subscriptions

[albrow]

Currently, we do not have a way to unsubscribe from GraphQL subscriptions either on the server side or the client side. This will result in increased resource usage over time due to maintaining some state for clients that are no longer active. Addressing this will come in two parts:

1. On the server side: Add support for detecting disconnects, GQL_STOP messages, and slow/inactive clients. This may require changes to https://github.com/99designs/gqlgen.
2. In the TypeScript GraphQL client: Add support for unsubscribing. This may require changes to the Apollo Client Core API since at the moment it doesn't look like unsubscribing is supported.