update: allow deprecated signatures only in the archive

update: allow deprecated signatures only in the archive

In April 2017, Oracle's jarsigner and Google's apksigner both switched to
considering any APK signature that uses MD5 as unsigned.  Any old build
is likely to have a MD5 signature.  This sets up the archive as the only
place where these "disabled algorithms" are allowed in the repo, and
marks any APK signed by a "disabled algorithm" as having a "known
vulnerability"

This also now automatically moves APKs with invalid signatures to the
archive section.

#323