How to remove excess files?

Some files are in folder but no longer in `content.json`

https://github.com/HelloZeroNet/ZeroNet/issues/164 looks right, but I need manually trigger the removal for all sites.

Read the linked issue #164 and compare the files present in a site folder with its content.json. Find the existing removal path and determine how it could be manually triggered for all sites. Done means obsolete files are removed safely without affecting files still listed in content.json.

索引モデルが issue の本文から書いたものです。