Notification improvements

[FelberMartin]

There are currently a few improvement ideas for enhancing the notifications or fixing bugs:

• The notifications currently only show a generic profile picture (probably created by the authorName) and does not use the Artemis specific profile pictures
  • Implementation hints: In CommunicationNotificationManagerImpl.buildMessagingStyle we would need to use person.setIcon(...) and previously store the image url in the CommunicationMessageEntity
• Reading a message does not automatically remove the notification (like eg in WhatsApp)
• When pressing "Mark as read" on a notification, the message still shows as unread when later entering the app
• Create an overview that lists all past notifications similar to iOS (planned by @julian-wls)