Move away from :[type]:[value]: custom props

Created by: rjw57

Now we're using our own database, we need not have the hacky :[type]:[value]: format for custom props. To avoid breaking our existing app, I suggest adding a new set of custom props prefixed by legacy_. E.g.:

{
  // ...
  custom: {
    // ... existing sms_... props
    legacy_media_id: "12345",
    legacy_... // etc
  }
}

Then, once we've migrated sms-webapp to understand them, we can just remove the logic which updates sms_... props and add deleting them to the tidyup job.