Introduce jquery formvalidator for com_users

#### Executive summary

This PR converts the form validation on com_users to use plain jquery (no mootools call on every form ).
Also NO MORE INLINE SCRIPTS!

#### Testing

1. Apply first PR #4888 (!important)
2. Apply this PR
3. In the admin area go to com_users and try to submit any form.

If no javascript errors are logged in your browser and the functionality remains the same your test is a pass in any other case please report the errors here

Can we please avoid this `$script .=` thing? You do it right later with:

```php
JFactory::getDocument()->addScriptDeclaration("
    // JS code here
");
```

So it's easier to modify it or move it to a js file.

Thanks!