po plugin: name rendered master pages like slave ones (page.html.LL)

po plugin: name rendered master pages like slave ones (page.html.LL)

Apache's content negotiation transparently redirects any old URL (page.html) to
the new one, depending on the client preferred language (i.e. a German browser
will be fed with page.html.de). Transition to this naming convention is then
really smooth.

This naming convention allows one to deliberately display the master page, even
if her browser is configured for another language.

Signed-off-by: intrigeri <intrigeri@boum.org>