Remove 'debug' from the list of stores we import (#1532)

Now that we log to disk, we no longer need the debug object store from
the Chrome export.

FREEBIE

js/backup.js

@@ -173,7 +173,7 @@
   function importFromJsonString(idb_db, jsonString) {
     return new Promise(function(resolve, reject) {
       var importObject = JSON.parse(jsonString);
-      var storeNames = _.keys(importObject);
+      var storeNames = _.without(_.keys(importObject), ['debug']);
 
       console.log('Importing to these stores:', storeNames.join(', '));