add /files/ and /item_files/ API endpoints

backend/authentication/serializers.py

@@ -0,0 +1,12 @@
+from rest_framework import serializers
+
+from authentication.models import ToolshedUser
+
+
+class OwnerSerializer(serializers.ReadOnlyField):
+    class Meta:
+        model = ToolshedUser
+        fields = ['username', 'domain']
+
+    def to_representation(self, value):
+        return value.username + '@' + value.domain