firefox: extension settings

[0xlua]

extension settings config in nix:

policies."3rdparty".Extensions.${extensionUuid} = {...};`

Extensions:

• uBlock
• tridactyl
• libredirect: see Firefox configuration via policies.json for NixOS Setup? libredirect/browser_extension#905
• bitwarden & linkding: these store secrets inside the options. I'm not aware of any way for extensions to read these from files. Hacky workaround would be to symlink the storage.json directly from a sops template.