Draft: implement shiny app shortcode

Implements a shortcode for external webapp/shinyapps via iframes. Also implements the according consent-frame for external content.

Important note: the iframe is constructed entirely by the javascript function after the consent button is triggered, i.e. no content is loaded beforehand, therefore no personal information (like IP) is sent before consent is given.

The Webapp can be embedded into a site with following shortcode (example: MikroSimulatoR from destatis):

{{< webapps url="https://shinymikrosimapp.azurewebsites.net/" accept_label="Accept the terms" accept_text="Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet." >}}

Also see video below:

Bildschirmaufzeichnung_vom_2023-11-29__09-34-44

This MR is marked as draft as we still need some styling for the button. I'll get around to this today or next week.