show 'send private message' button only when current user has at least one users in his/her favorite list
change to private_message/new page - if user access it from inbox, display a dropdown menu as recipients from the user's favorite list
record the source page when user navigate to private_message/new, so it could navigate back once new private_message is submitted
add pagination to private_message/index page
remove 'reply' button from private_message/index page
select recipient from dropdown list