Replace sign-addon usage in web-ext to support addons submission api

[eviljeff]

This tool needs to be updated to support the newer addons submission api on addons.mozilla.org. It might involve deprecating this project and starting afresh. Just logging the issue so there's some tracking.

[eviljeff]

Plan is to add support for the addon submission api directly in web-ext, initially as an opt-in setting. sign-addon will remain a dependency on web-ext for now until the submission api becomes the default.