Refactored login logic out into its own activity so that it can be dropped anywhere.  Also, force users to log in before upvoting comments

The description pretty much sums it up.  The one thing to note which is a little awkward is that we need to reload the comments once the user logs in so that we can get proper upvote urls.  It's a little bit strange, but I couldn't figure out a way to construct upvote URLs on the fly since they require a key.