3803 use email visibility to display only the shared information backend

[bosiraphael]

Closes #3803

[Weiko]

@bosiraphael actually there is nothing to validate here, I originally thought this was an input type. IsEnum won't help you there.
If you want a proper enum type for your graphql schema, you can declare your type like this

export enum TimelineThreadVisibility {
  Metadata = 'metadata',
  Subject = 'subject',
  ShareEverything = 'share_everything',
}

then register it like thi

registerEnumType(TimelineThreadVisibility, {
  name: 'TimelineThreadVisibility',
});