draw arbitrary notifications matching on date, versions from gh-pages

Per email, this'll display a notification with an arbitrary title and body from our gh-pages site at http://jazzido.github.io/tabula/notifications.jsonp. We can target users to see these notifications via version number and a start date and an end date. 

Dunno how often we'll want to use this, but it's nice to have it -- perhaps asking users to take a survey or something.

I don't see if that updates checkbox is hooked up to anything. We should hook it up to a setting to turn off notifications so that, if the user doesn't want notifications, they'll never ping GitHub's servers -- or an analytics server we hypothetically set up to see how many people are using our software based on the ping from the notifications feeds.

I also, just for the sake of separation of concerns, moved both the notification JS code and the recent files deletion code out of `index.html.erb` and into their own .js files.