Use websockets in CWS for notifications to contestants

Currently CWS gets notifications by polling every 30 seconds, and gets submission statuses by doing polling with exponential backoff (starting at 1 sec, delay multiplier about 1.5). Instead, we could use websockets to deliver the results as soon as they become available. As a demo of how effective this might be, currently for contests with a public scoreboard, you can see your score change on RWS before it updates in CWS (because RWS already uses websockets, at least when configured correctly).

This might also require adding some new rpc's to notify CWS of new submission results, but hopefully not too much work.

On the other hand, it would probably be a good idea to have a fallback for when websockets don't work (e.g. broken reverse proxy config), to make sure notifications aren't missed entirely.

Similarly, I think there's a bunch of polling in AWS which could also theoretically use websockets.

Start by tracing the existing polling in CWS and the websocket implementation in RWS, then compare the related polling in AWS. Define the RPCs needed to deliver submission results and notifications, and verify that a reliable fallback prevents missed notifications when websockets are unavailable.