Extend the core model an abstract timestamp object

The core model is extended with an abstract model called
TimeStampedModel. Future and existing models should
inherit from this model to include the default timestamp fields.

scipost/models.py

@@ -179,6 +179,19 @@ TITLE_CHOICES = (
 title_dict = dict(TITLE_CHOICES)
 
 
+class TimeStampedModel(models.Model):
+    """
+    All objects should inherit from this abstract model.
+    This will ensure the creation of created and modified
+    timestamps in the objects.
+    """
+    created = models.DateTimeField(auto_now_add=True)
+    latest_activity = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        abstract = True
+
+
 class Contributor(models.Model):
     """
     All users of SciPost are Contributors.
@@ -258,7 +271,7 @@ class Contributor(models.Model):
 
     def public_info_as_table(self):
         """Prints out all publicly-accessible info as a table."""
-        
+
         template = Template('''
             <table>
             <tr><td>Title: </td><td>&nbsp;</td><td>{{ title }}</td></tr>