Add Twig blocks around form rows to allow extensibility

This allows people to extend the template and make
certain small changes to it, without having to replace all of it.
Use the full row width for the privileges list

Certain privilege descriptions can be quite long.
Register templates at a custom namespace too

This makes it possible for a person to override a template
(by putting a directory on the path that matches it)
and to allow that same template to extend the original by doing:

    {% extends '@DevtureUserBundle/DevtureUserBundle/template.html.twig' %}

That's a solution to the so-called: "Overriding a Template that also extends itself" problem