Add "alert" banner to /

On days where we have special circumstances (e.g., school closings or even just a special assembly), it'd be nice to have a place to display extra information about the day. Bootstrap has an "alert" component which may be handy

http://getbootstrap.com/components/#alerts

@QuinnE Do you want to start taking a look at how we might do this? This would be a good way to get into the codebase and figure out how all the parts are working together and what would be required to put a banner up on the front page.