Added missing landmarks field in study group model and fixed serializer for users

stude/study_groups/migrations/0002_studygroup_landmark.py

@@ -0,0 +1,20 @@
+# Generated by Django 4.2.3 on 2023-09-24 10:59
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('landmarks', '0001_initial'),
+        ('study_groups', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='studygroup',
+            name='landmark',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, to='landmarks.landmark'),
+        ),
+    ]