Change type for private blog settings
Change type for private blog settings

refs #5614 and #5503

- update private blog type, including update to settings.edit
- switch order of populate settings & update fixtures + populate all settings

Private blog settings should not be returned by public endpoints
therefore they need a type which is not `blog` or `theme`.
`core` doesn't suit either, as those settings don't usually have UI
To resolve this, I created a new type `private` which can be used
for any setting which has a UI but should not be public data

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost