Notifications may be edited and deleted in completed assessment event

[elenaustyugova]

According to "REQF_adm_asss-005" and "REQF_adm_asss-012", if the assessment event is completed, then there should be no edit and delete buttons for event notifications.

Steps to reproduce:

1. Go to the details of the completed event having a notification

Actual result:
The edit button is present for event notifications. When editing and saving changes, error 403 is occurred (see screenshot)
The delete button is present for event notifications. When deleting notification, error 403 is occurred

Expected result: Edit and delete buttons are missing for event notifications.

Req ID: REQF_adm_asss-005, REQF_adm_asss-012