Add a notify action to the notifications component

Refs #3160
- gh-notifications component now takes an optional notify
  parameter.  If present it will be invoked as an action
  when a notification is added or removed.
- Add a data-notification-count attribute to the main container
  that tracks the number of "top" notification messages that
  are currently being shown.
Merge pull request #3314 from jaswilli/issue-3160

Add a notify action to the notifications component