Use next.js custom server

[Janpot]

Remove the next command for starting toolpad and instead use a custom server.

• this gives us more control over the process in which next.js runs, we can add a pipeline to build user applications and components.
• this gives us control over the http server next.js runs inside. This will allow us to run request handlers that are not controlled by next.js. We can serve the user application separately from the editor. e.g. we can run a separate webpack-dev-middleware for the user application.