Improve select2 configuration

This PR improve the select2 configuration.

To disable select2 on some `select` form element, set data attribute `data-sonata-select2="false"` to this form element.

    ->add('category', 'sonata_type_model',
        array(
            'attr'=>array('data-sonata-select2'=>'false')
        )
    )

To overload default functionality of `allowClear`, set data attribute ``data-sonata-select2-allow-clear="true"`` to enable ``allowClear`` or ``data-sonata-select2-allow-clear="false"`` to disable ``allowClear`` parameter.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1889, #1776
| License       | MIT