refactor modals and vote buttons

changes i've made: 
> +for logger out users
> - made the comment box appear on only last page of discussion (instead of every page)
> - put the authentication/ ask to join modal in the show page so it's only ever rendered once
>- a refactor of the vote buttons into a partial. 

note that the return_to does not take the users to the voting page, because the vote links needed to be disabled from redirecting. there might be a better solution