Fix gravatar loading.

- Also remove old requests from list when a new request for the same
  view can be fulfilled from the cache. Otherwise, the new image can be
  incorrectly overwritten by the loaded image later on
- Properly clean up list in case of an exception
Fix NPE in case a tab was selected before load finished.
Get rid of submenu of explore menu item.
Fix loading of blog items.
Tune some layouts.
Fix direct jump to 'following' page.
Improve gravatar loader performance.

- Don't kill worker thread while there are pending loads
- Index cache by gravatar ID instead of URL. URL host can change depending on
  load balancing.