Fixes for release publishing

[tronical]

Fix slintpad publishing and creation of versions.txt, as noticed during the 1.6.0 release process.

[ogoffart]

The current www-releases/slintpad folder has a few extra files such as 404.html script.js versions.txt. I don't know if they are necessary though.

[tronical]

These are the files right now:

404.html
LICENSE.md
assets
index.html
preview.html
script.js
versions.txt

@aurindam Is it correct that script.js and 404.html should come from somewhere else? If yes, where?

[tronical]

I pushed a change to preserve the 404/license/script.js files that are also present in the other top-level folders in www-releases.