Backend for implementing real-time issue assignees using GraphQL subscriptions

What does this MR do?

This adds a GraphqlChannel where we can execute GraphQL subscriptions via ActionCable.

This also adds the Issuables::AssigneesUpdated subscription which is currently triggered every time an issue's assignees change.

This does not include the frontend for consuming this subscription because we need to deploy this first so that we don't get errors with rolling deploys.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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