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
- updating config.example.js with commented-out flags and description


Once this is merged in, http://support.ghost.org/config still needs to be updated with the new config options
Having this tailored only to the privacy flag is misleading.  I'd assume that the `isDisabled` function on the Config module could be used for any config value.  If we want to only do this for privacy I'd prefer renaming this to something more specific such as `isPrivacyDisabled` so as to not consume the intent.