Filter out notifications with non-existent subjects
Filter out notifications with non-existent subjects

ref #1025 #1238. This should prevent the frontend from crashing when
opening the notifications menu, but we still need to make sure
notifications are deleted properly when subjects are deleted.

Luceos/core,Luceos/core,malayladu/core,datitisev/core,malayladu/core,flarum/core,datitisev/core,malayladu/core,datitisev/core,datitisev/core,flarum/core,malayladu/core,Luceos/core,flarum/core,Luceos/core