✨(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/models.py

@@ -793,6 +793,7 @@ class Document(MP_Node, BaseModel):
             "favorite": can_get and user.is_authenticated,
             "link_configuration": is_owner_or_admin,
             "invite_owner": is_owner,
+            "mask": can_get and user.is_authenticated,
             "move": is_owner_or_admin and not self.ancestors_deleted_at,
             "partial_update": can_update,
             "restore": is_owner,
@@ -958,6 +959,7 @@ class LinkTrace(BaseModel):
         related_name="link_traces",
     )
     user = models.ForeignKey(User, on_delete=models.CASCADE, related_name="link_traces")
+    is_masked = models.BooleanField(default=False)
 
     class Meta:
         db_table = "impress_link_trace"