Replacing deprecated security context

The `Symfony\Component\Security\Core\SecurityContextInterface` was deprecated
with symfony 2.6. All instances of this interface where replaced with
`Symfony\Component\Security\Core\Authorization\AuthorizationCheckerInterface`
or
`Symfony\Component\Security\Core\Authentication\Token\Storage\TokenStorageInterface`
depending on the use case.
All instances of `SecurityContext` where replaced as well.
This was done based on the blog post
http://symfony.com/blog/new-in-symfony-2-6-security-component-improvements