Search: handle department & municipality serialization in unit results

The issue was noticed when using `include=unit.department` in search
raised "Object of type Department is not JSON serializable" errors. Add
department and municipality serialization to the search results to fix
the issue.

services/search/api.py

@@ -193,6 +193,12 @@ def to_representation(self, obj):
                     representation["connections"] = UnitConnectionSerializer(
                         obj.connections, many=True
                     ).data
+                elif "department" in include_field:
+                    representation["department"] = DepartmentSerializer(
+                        obj.department
+                    ).data
+                elif "municipality" in include_field:
+                    representation["municipality"] = obj.municipality.id
                 else:
                     if hasattr(obj, include_field):
                         representation[include_field] = getattr(