Integrate CI emails into notification system

What does this MR do?

Integrate pipeline emails into the notification system. See #21930 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Remove pushers from pipeline emails service (because that's already covered by notifications)
• Tests
  • With notification setting
  • Pipeline transited to success or failed

What are the relevant issue numbers?

Closes #21930 (closed)

app/helpers/notifications_helper.rb

@@ -74,4 +74,13 @@ module NotificationsHelper
     return unless notification_setting.source_type
     hidden_field_tag "#{notification_setting.source_type.downcase}_id", notification_setting.source_id
   end
+
+  def notification_event_name(event)
+    case event
+    when :success_pipeline
+      'Successful pipeline'
+    else
+      event.to_s.humanize
+    end
+  end
 end