Malea: Add a jumbotron. And delete sign in button.

I added a jumbotron, so our sign in page doesn't look so bare. I also got rid of the sign up button. It is superfluous, because a user must sign in through facebook...there is no other way to access the site. Many other apps do this, so I don't think it is a problem. Plus, it makes the page a lot cleaner. Anyway, much more work to be done to style the page nicely. Will continue to work on that.