Add health endpoint to Dash framework

[ndrezn]

All Dash apps should expose a /health or similar endpoint which could be consumed by any platform running a Dash app to check whether the server is running or not.

In debug mode we show "Server" connected/disconnected so we might be able to track this using a similar mechanism, but this would be necessary even when debug mode is disabled.

[ndrezn]

This is useful if the app is authenticated or to detect if a runtime error killed the server vs. if the app was just slow to load because of some configuration.

[ndrezn]

It's a long execution chain, but you can see how Dash detects backend connected here:

dash/dash/dash-renderer/src/components/error/menu/DebugMenu.react.js

Line 103 in 9276a63

const connected = error.backEndConnected;