[view] return HTML5 doctype. Closes #2869426 We dropped xhtml support in 3.17 but we kept providing documents using the XHTML strict doctype, while our content wasn't conform to the DTD since we dropped proper namespaces definition as well as declaration of cubicweb DTD extensions.

usr/lib/python2.4/site-packages/cubicweb/
etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances