Improve support for subscriptions in GraphQL playground

[albrow]

Currently, subscription support in the GraphQL playground (enabled via ENABLE_GRAPHQL_PLAYGROUND=true) is quite limited. The UI only shows the first message received from the subscription and there is no way to see more without sending the subscription request again. Subscriptions are supposed to be supported by GraphiQL (the tool we are using) but I couldn't figure out how to get it working in the context of a static HTML and JS page without Webpack. See this conversation on the GraphiQL Discord channel for more context.