apply ContentNegotiatingViewResolver

add default view : MappingJacksonJsonView
but when user login, and get comment as json, the user's passwod is
open. it need to be hiden.
fix recommend, opposite button was not work

set ContentNegotiatingViewResolver
ignoreAcceptHeader to true
Change response of calling ajax - recommend, opposite, delete action

change ontrollers return type String to ActionResult to send as json by
MappingJacksonJsonView.
More obvious I think but javascript is little dirty and can't recognize
.json easily.