Re-enable language pack signature checks for Linux and Windows and double-check macOS

With https://bugzilla.mozilla.org/show_bug.cgi?id=1437942 Mozilla removed search engines from language packs which means one reason less for us to modify them which means in turn that it seems we can enable signature verification at least on Linux and Windows.

We should double-check whether we still need to have the bookmarks problem (#21879 (moved)) for macOS which would remain the only reason to not enable the signature check there.