Show loading wheel for initial SAY_READY action instead of READY button

[joffrey-bion]

At the very beginning of the game, the action type is SAY_READY because the server waits for all clients to say that they have setup all subscriptions.

This makes the UI display the old "READY" button for a very brief instant.

We could instead display a loading wheel with a message like "Loading..." (before sayReady call) or "Waiting for other players to join..." (after our own sayReadycall), the same way we handle the WAIT action:
#72