✨(backend) allow masking documents from the list view

Once users have visited a document to which they have access,
they can't remove it from their list view anymore. Several
users reported that this is annoying because a document that
gets a lot of updates keeps popping up at the top of their list
view.

They want to be able to mask the document in a click. We propose
to add a "masked documents" section in the left side bar where the
masked documents can still be found.

src/backend/core/factories.py

@@ -150,7 +150,7 @@ class DocumentFactory(factory.django.DjangoModelFactory):
         """Add link traces to document from a given list of users."""
         if create and extracted:
             for item in extracted:
-                models.LinkTrace.objects.create(document=self, user=item)
+                models.LinkTrace.objects.update_or_create(document=self, user=item)
 
     @factory.post_generation
     def favorited_by(self, create, extracted, **kwargs):
@@ -159,6 +159,15 @@ class DocumentFactory(factory.django.DjangoModelFactory):
             for item in extracted:
                 models.DocumentFavorite.objects.create(document=self, user=item)
 
+    @factory.post_generation
+    def masked_by(self, create, extracted, **kwargs):
+        """Mark document as masked by a list of users."""
+        if create and extracted:
+            for item in extracted:
+                models.LinkTrace.objects.update_or_create(
+                    document=self, user=item, defaults={"is_masked": True}
+                )
+
 
 class UserDocumentAccessFactory(factory.django.DjangoModelFactory):
     """Create fake document user accesses for testing."""