Some HTML files were importing iframe_api.js automatically by detecting the referrer document.
While this was done in a safe way (the map container does not use cookies), it is not
a best practice to load a script originating from document.referrer.
This PR solves the issue by using PHP to inject the correct domain name in the HTML files.
Adds a first version of an API to communicate between an iFrame opened by WorkAdventure and WorkAdventure itself.
The first API method is a method allowing to add messages in the chat, from the iFrame.
Comes with a test file.
