ignored frontend/dist/app/ + fixed param for post new content

.gitignore

 .idea/
-frontend/dist/app/html-document.app.js
+frontend/dist/app/
 frontend_app_html-document/dist/html-document.app.js
 frontend_lib/dist/tracim_frontend_lib.js

frontend/src/appFactory.js

         apiHeader: FETCH_CONFIG.headers // should this be used by app ? right now, apps have their own headers
       },
       idWorkspace,
-      idFolder
+      idFolder: idFolder === 'null' ? null : idFolder
     })
 
     emitEventApp = (type, data) => GLOBAL_dispatchEvent({ type, data })