Upload unsigned packaged add-on to dev & stage (#2870)
Upload unsigned packaged add-on to dev & stage (#2870)

- For dev & stage only:
  - Restore the *.xpi and *.rdf upload removed in #2651
  - Restore creation of /static/addon/latest redirect with add-on hash
  - Configure frontend app to install add-on from /static/addon/latest

Issue #2865
chuckharmston/testpilot,fzzzy/testpilot,lmorchard/idea-town,mozilla/idea-town,flodolo/testpilot,flodolo/testpilot,meandavejustice/testpilot,lmorchard/testpilot,lmorchard/idea-town-server,mozilla/idea-town,meandavejustice/testpilot,mozilla/idea-town,flodolo/testpilot,chuckharmston/testpilot,lmorchard/idea-town-server,lmorchard/testpilot,mozilla/idea-town-server,fzzzy/testpilot,mozilla/idea-town-server,lmorchard/testpilot,mozilla/idea-town,lmorchard/idea-town,flodolo/testpilot,mozilla/idea-town-server,fzzzy/testpilot,meandavejustice/testpilot,mozilla/idea-town-server,chuckharmston/testpilot,lmorchard/idea-town,lmorchard/idea-town,lmorchard/idea-town-server,lmorchard/idea-town-server,fzzzy/testpilot,chuckharmston/testpilot,lmorchard/testpilot,meandavejustice/testpilot