Add embed form id in parent div

[EmmanuelVella]

When using an embed form, currently it displays like this :

<div class="form-group">
    <div id="my_form_foo" class="form-group">...</div>
    <div id="my_form_bar" class="form-group">...</div>
</div>

This PR adds the form id to the top level div, selecting it is now easier :

<div id="my_form" class="form-group">
    <div id="my_form_foo" class="form-group">...</div>
    <div id="my_form_bar" class="form-group">...</div>
</div>