auto reload for port not found &| better control of auto open ports

[konne]

During the setup of software like ng serve the port is sometime already opened and gitpod opens the page,
but the app is still not available and this lead to the following screen.

The wish is that this page just has a small js script that reloads that page every 5 sec. So after the app is really loaded
that the app is shown.

Another feature would be that your backend is checking if the port is really reachable via HTTP with an 200 and only open the browser window after the app is reachable.

[konne]

@rohan-patra thanks for your PR on this issue #5938