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
function has been renamed to something more expressive