Delegate #key to parent when working with a GPG subkey

The content returned by #key is important when veryfying the signature,
so given we don't want to repeat it in the database for GPG subkeys we
need to delegate it to the parent.

app/models/gpg_key.rb

@@ -8,6 +8,7 @@ class GpgKey < ActiveRecord::Base
   sha_attribute :fingerprint
 
   belongs_to :user
+  belongs_to :parent, class_name: 'GpgKey'
   has_many :gpg_signatures
   has_many :subkeys, class_name: 'GpgKey', foreign_key: :parent_id, dependent: :destroy
 
@@ -48,6 +49,10 @@ class GpgKey < ActiveRecord::Base
     super&.upcase
   end
 
+  def key
+    parent_id? ? parent.key : super
+  end
+
   def key=(value)
     super(value&.strip)
   end