Fix entity error

Fix the following errors:

`The association Kayue\WordpressBundle\Entity\Post#user refers to the inverse side field Kayue\WordpressBundle\Entity\User#posts which does not exist.`

and

`The association Kayue\WordpressBundle\Entity\Comment#user refers to the inverse side field Kayue\WordpressBundle\Entity\User#comments which does not exist.`