✨(backend) add creator field on document and allow filtering on it

We want to be able to limit the documents displayed on a logged-in user's
list view by the documents they created or by the documents that other
users created.

This is different from having the "owner" role on a document because this
can be acquired and even lost. What we want here is to be able to
identify documents by the user who created them so we add a new field.

src/backend/core/api/serializers.py

@@ -150,6 +150,7 @@ class ListDocumentSerializer(BaseResourceSerializer):
             "abilities",
             "content",
             "created_at",
+            "creator",
             "is_favorite",
             "link_role",
             "link_reach",
@@ -161,6 +162,7 @@ class ListDocumentSerializer(BaseResourceSerializer):
             "id",
             "abilities",
             "created_at",
+            "creator",
             "is_favorite",
             "link_role",
             "link_reach",
@@ -181,6 +183,7 @@ class DocumentSerializer(ListDocumentSerializer):
             "abilities",
             "content",
             "created_at",
+            "creator",
             "is_favorite",
             "link_role",
             "link_reach",
@@ -192,6 +195,7 @@ class DocumentSerializer(ListDocumentSerializer):
             "id",
             "abilities",
             "created_at",
+            "creator",
             "is_avorite",
             "link_role",
             "link_reach",