Server status badge is broken

[teutoburg]

The ESO ETC server badge in the README needs to be fixed somehow. It used to simply check the base URL https://etimecalret-002.eso.org/observing/etc, but when the API was down recently, that still worked so the badge wrongly indicated a working server. I changed it to check the full https://etimecalret-002.eso.org/observing/etc/api/skycalc instead, but this always produces 500s, even if it's now working fine again. We should find a way to make that badge correctly correspond to the status of the server, i.e. show it as up when our skycalc requests go through, and as down when they don't. I do think the (working) badge is a useful addition for users to quickly check if the issue is on their end or not (the error msg would tell you so as well, but experience shows not everyone reads those...).