Move formactions wrapper into separate block

[SiliconMind]

This small change helps to add custom action without overwriting whole formactions block. Currently to add new action on edit form we have to rewrite whole formactions block. This small change allows this:

{% extends 'SonataAdminBundle:CRUD:edit.html.twig' %}

{% block formactions %}
    <button type="submit" class="btn btn-success" name="btn_my_action">My action</button>
    {{ parent() }}
{% endblock %}

[K-Phoen]

So, what's the status of this PR?

[SiliconMind]

@K-Phoen Don't ask me - the automated checks errors were not related to my code at all and I can't merge them because I don't have write access to the Sonata repo.

[K-Phoen]

I was more asking this to @rande or @pulzarraider :)

[pulzarraider]

@SiliconMind to make this PR more usable and without BC break I suggest you to leave the old block formactions at the same place. Instead of that please create new prefixed blocks:
sonata_pre_form_actions, sonata_form_actions and sonata_post_form_actions.

So we will have:

{% block formactions %}
   {% block sonata_pre_form_actions %}<div ...{% endblock %}
   {% block sonata_form_actions %}...{% endblock %}
   {% block sonata_post_form_actions %}</div>{% endblock %}
{% endblock %}

[pulzarraider]

Closing in favor of #2919