[FEATURE] Add support for custom html scripts for link surveys

[mattinannt]

Is your feature request related to a problem? Please describe.

Users would like to easily include Google Tag Manager / Google Analytics or Facebook / LinkedIn Pixels into their Link surveys.
Currently there is no way to do this.

Describe the solution you'd like

Allow custom HTML code injection (CSS, javascript,..) into the Link Survey pages. This should be done on a product level in the Look & Feel Settings (!IS_FORMBRICKS_CLOUD).

Describe alternatives you've considered

No response

Additional context

We should only allow this on self-hosted Formbricks instances since otherwise this imposes data privacy issues on Formbricks Cloud since we would need to update the data privacy policy for different link surveys and maybe even include cookie banners for this. To avoid this, we only allow this on self-hosting Formbricks instances.