meta: Make proxy more robust.

[showell]

We may want to divvy these up between Steve and Rohitt.

Proxy client:

• extract a really simple channel class for fetches
• handle errors from bad sends/fetches, even if it's just writing clear messages to the console for now (and specific widgets can always do smarter things, like add retry buttons)
• if server tells it the auth token is expired (TBD), handle appropriately (although maybe this is moot)
• handle broken websocket connection, and re-connect
• consider sending heartbeats over the websocket

Proxy server:

• implement logout
• have a login page (before authorize)
• at startup time, possibly sanity check a couple server endpoints
• handle broken websocket connection, log to console, and remove it from the list of clients

Auth tokens:

• see if we need to use the refresh token
• make sure we're really deleting the auth token on the server for logouts (as opposed to just killing the session)