Enable custom server entrypoints

This addresses and closes #428 (closed).

We've heard from many sources that the default server app.js (https://gitlab.coko.foundation/pubsweet/pubsweet/blob/master/packages/server/src/app.js) brings in a lot of things and is unwieldy in some situations. Ideally we'd reduce the number of things in app.js, but since it's hard-coded into server, it's difficult to play around with different designs without breaking the world.

I think this custom app approach will allow us to more quickly refine the default app.js, as well as enable more advanced use-cases (like using app.js for top level DI configuration, composition).

The way it works:

1. You can add a ./server/app.js file in your app's folder
2. You can configure a specific file in your config at pubsweet-server.app

The simplest app.js is something like this:

const configureApp = app => {
  app.get('/verify', (req, res) => res.send('hi'))

  // Actions to perform when the HTTP server starts listening
  app.onListen = async server => {
    // No-op
  }

  // Actions to perform when the server closes
  app.onClose = async () => {
    // No-op
  }

  return app
}

module.exports = configureApp

When you provide a custom app.js, you're very much on your own in terms of what the server provides. The only thing the server does for you is start itself and start listening on a configured port.

Feedback welcome!