Add migration to generate GpgKeySubkeys records and update signatures
user
```ruby
# See http://doc.gitlab.com/ce/development/migration_style_guide.html
# for more information on how to write migrations for GitLab.

class CreateGpgKeySubkeysForExistingGpgKeys < ActiveRecord::Migration
  disable_ddl_transaction!

  DOWNTIME = false

  class GpgKey < ActiveRecord::Base
    self.table_name = 'gpg_keys'

    include EachBatch
    include ShaAttribute

    sha_attribute :primary_keyid
    sha_attribute :fingerprint

    has_many :subkeys, class_name: 'GpgKeySubkey'
  end

  class GpgKeySubkey < ActiveRecord::Base
    self.table_name = 'gpg_key_subkeys'

    include ShaAttribute

    sha_attribute :keyid
    sha_attribute :fingerprint
  end

  def up
    GpgKey.each_batch do |batch|
      batch.each do |gpg_key|
        create_subkeys(gpg_key) && update_signatures(gpg_key)
      end
    end
  end

  def down
  end

  private

  def create_subkeys(gpg_key)
    gpg_subkeys = Gitlab::Gpg.subkeys_from_key(gpg_key.key)

    gpg_subkeys[gpg_key.primary_keyid.upcase]&.each do |subkey_data|
      gpg_key.subkeys.build(keyid: subkey_data[:keyid], fingerprint: subkey_data[:fingerprint])
    end

    # Improve latency by doing all INSERTs in a single call
    GpgKey.transaction do
      gpg_key.save!
    end
  end

  def update_signatures(gpg_key)
    InvalidGpgSignatureUpdateWorker.perform_async(gpg_key.id)
  end
end
```
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