Rename "client" to "subscription" in WebSocket RPC messages

[EliAndrewC]

As explained in these and other Sideboard docs, our WebSocket RPC messages have a "callback id" which identifies specific calls and and a "client id" which identifies a subscription. It's become clear to me from explaining this to people that "client id" is a terrible name for the id of a subscription, and it should really be "subscription id".

We can make this change in a backwards-compatible way:

• Update the Sideboard server code to check for both a subscription and a client field, defaulting to subscription.
• Update the Sideboard client code to set both fields, since Sideboard ignores extra fields and thus it won't hurt to have the redundancy.
• Eventually after all deploys have been updated, remove the client field entirely.