Add configurable dashboard templates

[Wharenn]

This PR add configurable dashboard templates using the Admin template system.

Usage :

sonata_admin:
    admin_services:
        sonata.dashboard.admin.dashboard:
            templates:
                view:
                    compose: 'MyBundle:Folder:customCompose.html.twig'
                    compose_container_show: 'MyBundle:Folder:customComposeContainerShow.html.twig'

[OskarStark]

please add a test for this new feature.

thank you

[soullivaneuh]

According to the new Sonata version management and next major release plan, this project has been refactored regarding branching and versioning.

If you see this message, your PR concerns a patch or a minor release and is not targeting the right branch.

So I'm closing this one, but don't see it as a refusal. If you think your work is still relevant and want to continue, feel free to reopen it on the right branch (e.g. the default one).

Regards.