Before this change, using [custom template for actions](https://symfony.com/doc/2.x/bundles/EasyAdminBundle/tutorials/custom-actions.html#custom-templates-for-actions) wasn't possible, as well as [collapsing actions](https://symfony.com/doc/2.x/bundles/EasyAdminBundle/book/configuration-reference.html#collapse-actions).

This PR makes it possible while keeping the [confirmation modal](https://symfony.com/doc/2.x/bundles/EasyAdminBundle/book/configuration-reference.html#collapse-actions).

It introduces a new `action.html.twig` template, which keeps the confirmation behavior. This is the new template to use when creating a custom template.

This PR also changes the `_actions_template` template selection, giving us the ability to use the collapsed version of actions.

Side note: the new action template differs slightly from the original one in the way it deals with translation and doesn't use the same CSS classes for the action icon. This was left unchanged.