add requireCloseClasses and changed message to tripple stache

This PR does basically 2 things. 
1) I wanted the ability to put links in notifications.  To achieve that I needed the notifications to not close when the user clicks anywhere.  So I left the notification get created with a list of classes, one of which must be the click target.  Thus I can make the notification close when the user clicks the link or the X, but nowhere else.
2) In order to put links (or any HTML) in the message, I put changed {{message}} to {{{message}}.  I screwed around with using Spacebars.SafeString, but that object doesn't serialize across Sessions.  Maybe we should make it a EJSON type so it does, but I guess I'm unsold on if this really matters.  Since it's all on the client anyway and users can create notifications for other users, what's the harm?  Am I overlooking some security thing here?  Or perhaps there's a better more simpler way to solve this?
Please add curlybraces. 

Also maybe ```!_.intersection(this.requireCloseClasses, event.target.classList).length``` would be better suited here because that will also work with multiple classes on the event.target