This commit enables a "read all" feature for notifications.

Classes:
- NotificationController: Add method readAll()
- Notification: Add method markAllAsRead()

Templates:
- menuitem.scala.html: Add div for readAll

CSS:
- htwplus.css: Increase max-height for hp-dropdown-search

Other:
- messages: Add i18n for show all and flash
- routes: Add route for NotificationController::readAll()