Added the following Javascript hooks:
* ext.echo.notifications.beforeRender: Firing before a group of
notification widgets are rendered, whether in the popup, in
the special page, or in a cross-wiki bundle (which requires
async loading)
* ext.echo.badge.countChange: Fired when the badge count changes
with the notification type, count and the label count for
display purposes.
* ext.echo.popup.onInitialize: Fired when the popup is opened and
after notifications were fetched, with the context of the popup
notification type.
* ext.echo.special.onInitialize: Fired when the special page is
ready and notifications were fetched. Note that it will be fired
whenever the special page is updated with notifications list,
as well, like when changing filter, remote wiki or pagination.
The hooks were also documented in hooks.txt
Bug: T146296
Change-Id: Ie3dc97f97e8d1f90b67f62fcdc65dd29cb379aad
