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
Seems like a pretty good idea to me.  It will probably need to be adapted to handle nested configuration options at some point but we can cross that bridge when we come to it.

My one (dumb) comment would be to change `that = this` to `self = this` so it's consistent with the rest of the project.