XWIKI-8600: AppWithinMinutes: No error displayed when trying to add an entry already existing.

The previous pull request: https://github.com/xwiki/xwiki-platform/pull/165 (I have created a new PR because I has some problem on my local repo).

* Add an AJAX call to check if the entry already exists and display a message if so.
* Check the view & edit right on the page if it already exists
* If the user has only the 'view' right, then see the document instead of editing it.
* Add a document suggest.