Add fallback nginx to serve "something went wrong" page

[mrnicegyu11]

The maintenance page should always run as a kind of fallback/404 as a last priority. We tried running an oSparc with the maintenance page up with prio=1 (lowest prio), but the e2e/p2e showed problems. I closer look at all traefik routes and a fine-tuning are required to make this work properly.

[YuryHrytsuk]

@mrnicegyu11 Do we really want to show maintenance page in case webserver is down? I think that we better show maintenance page when we really do maintenance in which case we can always set up it explicitly. In case we show maintenance page every time the webserver cannot be reached. It can make users think that all fine and the maintenance is happening while we are actually have no clue that something is broken?