Initialise global message

- Create new class to manager global message. My idea is that this class permit to manage audio or text message.
 - Update html to have main content id and inject html in this.
 - Create front event to receive startMessage and stopMessage.
 - Delete impoted variable not used.

front/src/WebRtc/HtmlUtils.ts

@@ -7,4 +7,14 @@ export class HtmlUtils {
         // FIXME: does not check the type of the returned type
         return elem as T;
     }
+
+    public static removeElementByIdOrFail<T extends HTMLElement>(id: string): T {
+        const elem = document.getElementById(id);
+        if (elem === null) {
+            throw new Error("Cannot find HTML element with id '"+id+"'");
+        }
+        // FIXME: does not check the type of the returned type
+        elem.remove();
+        return elem as T;
+    }
 }