Prevent Ikiwiki from formatting the Unsafe Browser warning pages.

This is a follow-up on 08259afa.

Due to #14962 we really only want the simple text-only HTML page
without all the Ikiwiki formatting added. I'm very ignorant of
Ikiwiki, so this hack is the best I could come up with so I can
quickly rebuild Tails 3.3. I'm also sloppy: I verified my assumption
that those .html pages would be left unprocessed...

Refs: #14962