Server Messages unpublish

* changes unpublish_on to optional value
* simplifies fetching of server message news

news/migrations/0003_auto_20230324_0720.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.1.3 on 2023-03-24 06:20
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('news', '0002_auto_20220114_0936'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='servermessage',
+            name='unpublish_on',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]