module Devise::Mailers::Helpers

subject: '...'
confirmation_instructions:
mailer:
devise:
en:

If one does not exist, it fallbacks to ActionMailer default:

user_subject: '...'
confirmation_instructions:
mailer:
devise:
en:

based on the current mapping:
Set up a subject doing an I18n lookup. At first, it attempts to set a subject

def subject_for(key)
  I18n.t(:"#{devise_mapping.name}_subject", scope: [:devise, :mailer, key],
    default: [:subject, key.to_s.humanize])
end