text: Title: Notification Rules for unread reports
  Body: ### Proposal
  
  At the moment the notifications for unread reports are sent to all recipients of a context, also if one of them as already read the report.
  Ex. Context with 3 recipients: A, B, C.
  If A has read the reports, notifications should not be sent to B and C because in this way these two recipients get spammed while the report has been already read by A.
  
  Then, it would be better to configure this feature also on the sub-sites and not only on the main one,  because it is intended to have sub-sites that may want to use different set up.
  
  ![image](https://github.com/globaleaks/GlobaLeaks/assets/113996868/f978dcae-1e6c-484f-9218-599c499b61d5)
  
  
  
  
  ### Motivation and context
  
  - Reduce spam email notifications
  - Grant possibility to configure this feature in different ways for single sub-sites