Make migration an asynchronous task

[WithoutPants]

This addresses a long standing issue where running a schema migration that takes a long time can time out the connection if running through a reverse proxy.

This changes the migration operation to be a job-based task rather than synchronous operation. This is a breaking change to the migrate mutation - it now returns a job ID immediately. It is left to the client to monitor the status of the migration job.

The UI is changed to monitor the job, and display a progress bar with the current migration task being performed.

Job monitoring nominally uses the websocket connection, and it's possible that this connection is not present. The lack of a websocket connection would prevent job monitoring, so if the websocket connection isn't available, then it falls back to polling every second.

To receive errors when the migration task fails, the Job type has been expanded to include an error field, and a new status FAILED is available.

This PR also includes a connection monitor component which detects if the websocket connection fails to establish and displays a toast error. If the websocket connection is later re-established, then a toast message is shown.