Instead of statically listing the redirections move to a dynamic model.
We move the existing _extras/.htaccess to _templates/htaccess so we have
some control and safety of what goes in there. Connect 'build-finished'
from _exts.deliverables.py to trigger generating the redirects. Doing
so here avoids re-reading the data as deliverables.py ahas already done
that for us.
Change-Id: If6bd59fd478593a84ebcedc3a50af3720d620d3c