With this PR a new text field will be added that allows setting custom
html code to the `<head>` of a status page.
The implementation will be similar to
https://github.com/louislam/uptime-kuma/pull/2567/files, but with a
multi-line text field and without escaping any special chars.
For security reasons the env var `UPTIME_KUMA_ALLOW_CUSTOM_HTML` must be set to `1` to enable this feature.
This will allow tracking with most analytic platforms and has been
requested several times.
Closes#2818
