Have an underlay for the new Firefox extension

Originally created by @sajolida on #14934 (Redmine)

I propose that we use a signed but unlisted add-on to distribute our Tails Verification extension.

See https://developer.mozilla.org/en-US/Add-ons/Distribution

People would download the XPI from our website instead of from AMO. The benefits is that we don’t have to do through their employee review on submission and update.

So we need a dedicated Git repo and underlay to host these binaries, like we did for UUI.

Parent Task: tails#12328 (closed)