Use the common attribute "_id" in JS to retrieve the item id.

These changes prevent error when user make a link to an item (a page or a media in that case).

The JS atribute "_id" is better to use because it's set in a deepest API level than the "id" attribute. So it's set in much case.

These changes correct : 
* Make a link to a new page in a wysiwyg
* Make a link to a new media in a wysiwyg
* Add a new media directly into a wysiwyg (the system returned that the media does not exist)
* A freshly added media wasn't selected in the media selector.