Provide signed releases on GitHub

[SebastianZ]

https://developer.mozilla.org/en-US/Add-ons/Distribution describes how to sign an extension, so it can be installed without having to set xpinstall.signatures.required preference to false.

This would allow providing signed releases on GitHub (additionally to the ones on AMO).

Sebastian

[janodvarko]

https://bugzilla.mozilla.org/show_bug.cgi?id=1230073

Honza