adding config flags to control all items mentioned in PRIVACY.md

closes #3241
- in config.js, the `privacy` attribute holds all privacy-related flags
- `privacy.useTinfoil: true` disables everything (equivalent to setting all flags to false)
- added helper function to core/server/config/index.js to checking privacy flags
- added helper function to core/server/config/index.js to show warning about deprecated items


Once this is merged in, http://support.ghost.org/config needs to be updated with the new config options as well as a link placed in `PRIVACY.md` to indicate how things can be disabled. Below is a sample writeup of how to use the new privacy flags (a potential for the documentation site?)


> Flags to better privacy controls and usage of 3rd party services - see https://github.com/TryGhost/Ghost/blob/master/PRIVACY.md for more details.
> Setting an individual privacy flag to `false` indicates that that particular 3rd party feature should not be utilized.
> If a particular flag is not present, then than it will be assumed to be `true`.
> There is also a special privacy flag called `useTinfoil`, which if set to `true` then Ghost will ignore all other privacy flags and disable the use of all 3rd party services as well as automatic updates (this is equivalent to setting each privacy flag to `false`).
```
privacy: {
    useTinfoil: true,
    useUpdateCheck: true,
    useGoogleFonts: true,
    useGravatar: true,
    useRpcPing: true
}
```