Implement a stale-while-revalidate strategy for the Pages popup

[alexiglesias93]

Right now every time we open the Pages popup we rescan the whole site's sitemap to find out all the pages.
This often leads to a few seconds of waiting each time we open the popup:

We could improve this experience by adopting a stale-while-revalidate strategy: when opening the popup we display the cached pages (if previously fetched) while we re-load them in the background. If there are any changes after loading, we simply re-render the pages tree.
We could display a loading spinner at the top of the popup indicating that we are refetching the content in the background: