Manual role notification

Besides #250 (closed), there should be a way to manually send a message (notification) to all users with a specific role. For this notification, the message should be specified with the action. The notification would be sent to all users having the selected CourseSpecificRole.

• Add partial with button and (plain) text field, include on course edition management page
• Add route and controller action to create the notification
• Add notification in en.yml, same as automatic notifications but use different name such that it becomes a separate setting.