Plugin routes fail if Stud.IP is behind a proxy (FPM other otherwise)

[tgloeggl]

The meetings plugin uses SLIM as ist PHP-Framework. There is an known issue, where SLIM does not correctly identify to which protocol to redirect. It is then trying to redirect from HTTPS to HTTP, which fails in modern browsers do to mixed content policies.

See https://discourse.slimframework.com/t/request-returns-http-instead-of-https for further details.

While it is possible to fix this in the environment (in NGINX of $_SERVER['HTTPS'] = 'on'; in Stud.IPs config/config_local.inc.php) it might be a good idea to have a fallback in the plugin as well to prevent hard to track errors.