Fix disablement of "search" and "clear" buttons in user/group pages

On the users and groups management pages, a JavaScript code is used to prevent users from clicking multiple times on the "Search" and "Clear" buttons, to prevent multiple calls while a request is being processed.

This code is place on the "onsubmit" attribute of the form, but is not effective anymore, and the corresponding WebDriver test uses a non-ajaxified click, producing invalid requests.

This issue has already been experienced when working on NXP-18117 (linked to NXP-13651) and has come back when working on NXP-17690.

=> fix the behaviour and use button.click() instead of button.submit() in webdriver tests (to send a proper ajax request instead of a non-ajax one).