Release candidate of the selectable unsubscription

Implemented the resubscription process - i.e. pre-filling in the form when the subscription link is clicked in the unsubscription notice.

lib/models/settings.js

@@ -15,6 +15,8 @@ function listValues(filter, callback) {
         filter = false;
     }
 
+    // TODO: It would be good to cache the settings. It feels awkward to always go to DB to retrieve something what is essentially a constant
+
     filter = [].concat(filter || []).map(key => tools.toDbKey(key));
 
     db.getConnection((err, connection) => {