Update all loggedInUser access to be dynamic (bug 1066511)

The logic here seems ok but I don't think you need to manipulate the DOM. It seems heavyweight. Why not set ``logged_in_email`` as a Session property, defaulting to what's int the DOM, like this:

    var SessionModel = Backbone.Model.extend({
      defaults: {
        email_hint: utils.bodyData.loggedInUser
      }
    }

then reference that variable when you need it. Delete it on logout. It's not much different but I guess there would be a slight speedup since you wouldn't touch the DOM. It seems cleaner to encapsulate this data in the session model to me as well.