closes https://github.com/TryGhost/Ghost/issues/7305
The `location` property of server-side notifications was being ignored by the client and so wasn't being used to de-duplicate alerts.
- adds `key` attribute to `Notification` model
- adds a serializer for notifications that renames the `location` key sent by the server to `key`
- set the default background color of alerts to white so that alerts with no `type` set do not inherit the background color (useful in Ghost Desktop which has a transparent background color set)
