adds acts_as_voter to user model so that it can utilize acts_as_commentable methods
 upvote button only displays if user has not already upvoted the trip, displays a message if the user has upvoted the trip
adds a list of trips the user has upvoted to their profile page
Merge branch 'master' of https://github.com/pocus/mondiale
 users can now remove their upvoting of a trip and the appropriate message will display