Use process events instead of polling for INK conversion

A minimal version of !9 (closed) that keeps the current flow (the client uploads a file and waits for the response), but uses events from INK's Pusher/Slanger processing channel instead of polling until the processing is finished.

The app config looks like this (with example values):

{
  "pubsweet-component-ink-backend": {
    "pusher": {
      "appKey": "abc123",
      "wsHost": "100.100.100.100",
      "wsPort": 8080,
      "httpHost": "100.100.100.100",
      "httpPort": 4567
    }
  }
}