FEATURE: Improve backup stats on admin dashboard

* Dashboard doesn't timeout anymore when Amazon S3 is used for backups
* Storage stats are now a proper report with the same caching rules
* Changing the backup_location, s3_backup_bucket or creating and deleting backups removes the report from the cache
* It shows the number of backups and the backup location
* It shows the used space for the correct backup location instead of always showing used space on local storage
* It shows the date of the last backup as relative date

lib/backup_restore/s3_backup_store.rb

@@ -24,7 +24,11 @@ module BackupRestore
 
     def delete_file(filename)
       obj = @s3_helper.object(filename)
-      obj.delete if obj.exists?
+
+      if obj.exists?
+        obj.delete
+        reset_cache
+      end
     end
 
     def download_file(filename, destination_path, failure_message = nil)
@@ -38,6 +42,7 @@ module BackupRestore
       raise BackupFileExists.new if obj.exists?
 
       obj.upload_file(source_path, content_type: content_type)
+      reset_cache
     end
 
     def generate_upload_url(filename)
@@ -100,5 +105,9 @@ module BackupRestore
         SiteSetting.s3_backup_bucket
       end
     end
+
+    def free_bytes
+      nil
+    end
   end
 end