add nullable document_count field to Notifications

intention is for this to be null, 1, or many, based on how many
documents were linked to within the message. nullable column, so that it
doesn't require a lengthy access exclusive lock on the table when
creating.

app/models.py

@@ -1407,6 +1407,8 @@ class Notification(db.Model):
 
     reply_to_text = db.Column(db.String, nullable=True)
 
+    document_download_count = db.Column(db.Integer, nullable=True)
+
     postage = db.Column(db.String, nullable=True)
     CheckConstraint("""
         CASE WHEN notification_type = 'letter' THEN
@@ -1683,6 +1685,8 @@ class NotificationHistory(db.Model, HistoryModel):
         END
     """)
 
+    document_download_count = db.Column(db.Integer, nullable=True)
+
     __table_args__ = (
         db.ForeignKeyConstraint(
             ['template_id', 'template_version'],