Fixed invalid entities ("inversedBy" parameter set to null)

Fix for alerts (on the Symfony profiler toolbar) about invalid entities in SonataPageBundle. The given messages are :

Application\Sonata\PageBundle\Entity\Page :
The field Application\Sonata\PageBundle\Entity\Page#children is on the inverse side of a bi-directional relationship, but the specified mappedBy association on the target-entity Application\Sonata\PageBundle\Entity\Page#parent does not contain the required 'inversedBy=children' attribute.
The field Application\Sonata\PageBundle\Entity\Page#blocks is on the inverse side of a bi-directional relationship, but the specified mappedBy association on the target-entity Application\Sonata\PageBundle\Entity\Block#page does not contain the required 'inversedBy=blocks' attribute.
The field Application\Sonata\PageBundle\Entity\Page#sources is on the inverse side of a bi-directional relationship, but the specified mappedBy association on the target-entity Application\Sonata\PageBundle\Entity\Page#target does not contain the required 'inversedBy=sources' attribute.
Application\Sonata\PageBundle\Entity\Block : 
The field Application\Sonata\PageBundle\Entity\Block#children is on the inverse side of a bi-directional relationship, but the specified mappedBy association on the target-entity Application\Sonata\PageBundle\Entity\Block#parent does not contain the required 'inversedBy=children' attribute.