Integrate iframe-resizer our iframe controller and replace custom resize code

To make scrollbars in iframes disappear we have some custom javascript code that gets injected in every html page loaded in an iframe (iframe.js). The code is old and does not work for all cases. Luckily, there is a library iframe-resizer that does exactly this.

We integrated the library already in the card2brain course element and made good experiences. The advantage of the solution is that users who create their own html5 apps can control the resize behaviour from within the iframe. iframe-resizer is kind of a standard library used by many other projects.