Improve ajax call to build composer page url slug

* 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
Unify form submission event handlers

Let's have just one function that handles the submission validation
Merge pull request #1395 from mlocati/fix-slug-ajax-build-composer

Improve ajax call to build composer page url slug