Notifications Dropdown infinite scrolling

I don't think there was an issue for this but i felt compelled to implement it after working on issue #5106.
Whenever the user scrolls to the bottom of the notifications dropdown it will load 5 more notifications until there are none left to show
Should `self.notifications.length % 5 === 0` prevent loading more notifications when there are no additional notifications? That doesn't work when the number of total notificatons is divisible by 5. You could instead check if `notificationsLoaded <= self.notifications.length`.