[test] Add a test ensuring new cube built from skeleton is flake8-compliant And make skeleton files actually flake8-compliant henceforth.

[skeleton] Ship a tox.ini file upon new cube

[web] fix headers on 304 responses (closes #11466875) The response MUST include the following header fields: [...] - ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5 Unfortunately we weren't copying headers from what would have been the 200 response, but from the request.