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.tinfoil: 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 item to `true` indicates that it should not be utilized.
> Settings `privacy.tinfoil` to `true` will ignore all other flags and enable ALL privacy options (same as setting each on to true).
```
privacy: {
    tinfoil: true,
    noUpdateCheck: true,
    noGoogleFonts: true,
    noGravatar: true,
    noRpcPing: true
}
```
Absolutely agree.