Use short dates when selection notifications for deletion.

This means we will retain notifications for a full week and not
delete records that are 7 x 24 hours older than the time of the run of
the deletion task.

Also the task only needs to run once a day now, so I have changed
the celery config for the deletion tasks.

app/models.py

@@ -1,7 +1,10 @@
 import uuid
 import datetime
 
-from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.dialects.postgresql import (
+    UUID,
+    JSON
+)
 
 from sqlalchemy import UniqueConstraint
 
@@ -410,3 +413,12 @@ class TemplateStatistics(db.Model):
         unique=False,
         nullable=False,
         default=datetime.datetime.utcnow)
+
+
+class Event(object):
+
+    __tablename__ = 'events'
+
+    id = db.Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    type = db.Column(db.String(255), nullable=False)
+    details = db.Column(JSON, nullable=False)