Client & Server side validation for posts per page
Client & Server side validation for posts per page

closes #839

- caused a 500 error

JohnONolan/Ghost-Admin,TryGhost/Ghost-Admin,airycanon/Ghost-Admin,acburdine/Ghost-Admin,JohnONolan/Ghost-Admin,kevinansfield/Ghost-Admin,dbalders/Ghost-Admin,acburdine/Ghost-Admin,dbalders/Ghost-Admin,kevinansfield/Ghost-Admin,TryGhost/Ghost-Admin,airycanon/Ghost-Admin