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.userTinfoil: 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
Merge pull request #3874 from morficus/issue-3241

adding config flags to control all items mentioned in PRIVACY.md
[#75777002] have Ghost recognize middleware:true in config.js, alternative to server:{}
[#75777002] have Ghost respond to middleware:true by not having Express listen for requests