Use `bbc` instead of `to` to hide list of recipienta

BulkUserEmailByLocale.rb

users_by_locale.each do |locale, locale_users|
  I18n.with_locale(locale) do
     mail(
        bcc: locale_users.map(&:email),
        subject: I18n.t('user_mailer.new_follower.subject')
     )
  end
end

private

def users_by_locale
  @users.group_by { |user| user.locale }
end

@glaszig, good point — I've updated the gist to use bbc instead of to. Thanks for bringing it to my attention.