Use abstract ModelWithCreatedUpdatedFields for Host model

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

program/models.py

@@ -91,9 +91,9 @@ class ModelWithCreatedUpdatedFields(models.Model):
     """
 
     created_at = models.DateTimeField(auto_now_add=True)
-    created_by = models.CharField()
+    created_by = models.CharField(max_length=150)
     updated_at = models.DateTimeField(auto_now=True)
-    updated_by = models.CharField()
+    updated_by = models.CharField(max_length=150)
 
     class Meta:
         abstract = True
@@ -175,7 +175,7 @@ class Language(models.Model):
         return self.name
 
 
-class Host(ModelWithImageFields):
+class Host(ModelWithImageFields, ModelWithCreatedUpdatedFields):
     name = models.CharField(max_length=128)
     is_active = models.BooleanField(default=True)
     email = models.EmailField(blank=True)