[Refactor] Move invitation code to the Event model

calsocial/models.py

@@ -417,6 +417,23 @@ class Event(db.Model):
 
         return url_for('event_details', event_uuid=self.event_uuid)
 
+    def invite(self, inviter, invited):
+        """Invite ``invited`` to the event
+
+        The invitation will arrive from ``inviter``.
+        """
+
+        invite = Invitation(event=self, sender=inviter, invitee=invited)
+        db.session.add(invite)
+
+        notification = Notification(profile=invited,
+                                    actor=inviter,
+                                    item=self,
+                                    action=NotificationAction.invite)
+        db.session.add(notification)
+
+        return invite
+
 
 class UserSetting(db.Model):  # pylint: disable=too-few-public-methods
     """Database model for user settings