Fix multiple registration of tinyMCE editor init on ajax requests

For each tiny mce editor, an initialization is registered on the ajaxSubmit event of the parent form. As a result, after an ajax render, an editor might be initialized many times.

This can significantly slowdown the load time of a page.

This is particularly visible on a list widget containing a html editor. Add many editors and you figure out that, after each click on add, there is an increasing time where you can see the page is loading