Add timestamp to icon filenames

There can be race condition when a background job is supposed to delete
an icon while there is a new one being attached. Also, this encodes the
date/time when the icon has been added, for inspection and convenience.

app/services/app_catalog_manager/update_metadata.rb

@@ -42,8 +42,8 @@ module AppCatalogManager
       else
         download_url = "#{@app.url}/#{icon["src"].gsub(/^\//,'')}"
       end
-      filename = "#{attachment_name}.png"
-      key = "web_apps/#{@app.id}/icons/#{attachment_name}.png"
+      filename = "#{attachment_name}-#{Time.now.to_i}.png"
+      key = "web_apps/#{@app.id}/icons/#{filename}"
 
       begin
         tempfile = Down.download(download_url)