store installDate and updateDate

background/storage.js

@@ -424,7 +424,7 @@ function saveStyle(style) {
           return style;
         }
         codeIsUpdated = !existed || 'sections' in style && !styleSectionsEqual(style, oldStyle);
-        style = Object.assign({}, oldStyle, style);
+        style = Object.assign({installDate: Date.now()}, oldStyle, style);
         return write(style, store);
       });
     } else {

background/update.js

@@ -127,6 +127,7 @@ var updater = {
 
     function maybeSave(json) {
       json.id = style.id;
+      json.updateDate = Date.now();
       if (styleSectionsEqual(json, style)) {
         // JSONs may have different order of items even if sections are effectively equal
         // so we'll update the digest anyway