Move `cubicweb.tags` to cube `web`
https://forge.extranet.logilab.fr/cubicweb/cubicweb/-/blob/4a7c8e33a6039fcd823008a83f336fa1a9cbc7d1/cubicweb/tags.py
It seems to be a remaining piece that seems to have now its location in the cube web
as it is very likely to be useless without it.