cubicweb #3276389 fix i18n headers in skeleton cube [validation pending]

Emacs (at least) needs a PO-Revision-Date header to be happy.

prioritynormal
typebug
done in3.18.0
load0.100
load left0.000
closed by#e148583db39e [skeleton i18n] Add PO-Revision-Date header to make emacs po-mode happy
patch[skeleton] change i18n headers to make emacs po-mode happy (closes #3276389) [applied]