Restructured public settings cache hash

refs https://github.com/TryGhost/Team/issues/509

- Flipping around key/newKey pair allows to map multiple keys to the same field in the settings
- This becomes handy when there's a need to deprecate a field. For example, we are about to introduce a 'locale' setting which would need to map to 'lang' db key, with current structure it's impossible to have many:1 mapping because it's impossible to have duplicate keys in the JS object ("hash")

core/server/services/settings/cache.js

@@ -99,7 +99,7 @@ module.exports = {
     getPublic() {
         let settings = {};
 
-        _.each(publicSettings, (newKey, key) => {
+        _.each(publicSettings, (key, newKey) => {
             settings[newKey] = doGet(key) || null;
         });