Move notifications from ajax into MessageBus push

This makes so the notifications dropdown doesn't wait for an ajax response to open per @SamSaffron's suggestion. Instead, notifications are pushed down in a MessageBus.

I tested the following:
1. Notifications appear upon logging in.
2. New notifications are pushed to the dropdown when the dropdown is already open.
3. The unread notifications count clears upon clicking the dropdown.
4. Unread notifications count stays the same after logging out then logging back in.
 
Thoughts? I really appreciate any code review suggestions.