Improve ajax call to build composer page url slug

As promised in #1340, here's the improvement for slug creation in composer:

* Disable form submission until the ajax request has finished
* Don't do ajax request if page name is the same as the previous ajax request
* Take in account the ajax response (success of fail) only if it's for the last ajax request

Bug-tracker: http://www.concrete5.org/developers/bugs/5-6-2-1/url-slug-suggestion-is-too-slow-when-adding-new-pages/