Change the frontend probe URL to /about

The route handling the original URL, /, does more than just return as simple response. It talks to the backend and prepares some statistics as it shows a worker load graph and the number of users and projects on this instance. Using it as a liveness/readiness/startup probe URL not only results to the frontend being killed when it becomes more sluggish than usual, but also creates unnecessary load on the backend.

On the contrary, /about does none of that, and is perfect for a probe URL.