add rake task for creating initial notifications

lib/tasks/one_off.rake

@@ -0,0 +1,14 @@
+namespace :one_off do
+  desc "Create a notification for all users telling them about the new notifications"
+  task notifications_announcement: :environment do
+    User.all.find_each do |user|
+      user.notifications.create(
+        message_html: '<div>A new feature is now available:</div><div class="blue-text text-darken-3">Notifications on Notebook.ai</div>',
+        icon:         'favorite',
+        icon_color:   'red',
+        happened_at:  DateTime.current,
+        passthrough_link: 'TODO'
+      )
+    end
+  end
+end