correctly implement websocket PING/PONG

The client has a hacked heartbeat implementation which sends empty text messages ^[1] to the server where they're unceremoniously dropped. ^[2] This is needed to make sure proxies don't drop the connection when the socket is inactive.

This implementation is redundant with the WebSocket spec's own heartbeat mechanism PING/PONG. ^[3] We can achieve achieve the same result by adding a broadcast timer to the server to emit PINGs and correctly implementing PING/PONG in our message handler.

This will allow the web service to dictate heartbeat interval based on the requirements of the hosting environment, enable omitting heartbeats for recently active clients, avoid filling the browser's WS frame inspector with garbage, and make the server more compliant with the WS spec.

Start with src/actions/websocket.js around the existing heartbeat and api/src/WSService.cpp around the server message handling. Trace the proposed broadcast timer and PING/PONG flow, then verify that inactive connections receive protocol heartbeats without empty text messages and that recently active clients can omit them.