Add ability to negotiate an application version number #2323

Feature Request

Description of Problem:

We would like to support blue/green deployments, but need the ability to force the frontend to reload if the backend has incompatible changes.

Potential Solutions:

1. Add a version number to the login response message, that the UI can use to optionally abort and reload the application.

2. Pre-emptively send the version in a new message, immediately after opening the websocket.