Remove redundant database column domains.registered_at

Alias `Domain#registered_at` to `created_at`

Closes #1442

app/models/domain.rb

@@ -18,6 +18,7 @@ class Domain < ApplicationRecord
   alias_attribute :on_hold_time, :outzone_at
   alias_attribute :outzone_time, :outzone_at
   alias_attribute :auth_info, :transfer_code # Old attribute name; for PaperTrail
+  alias_attribute :registered_at, :created_at
 
   # TODO: whois requests ip whitelist for full info for own domains and partial info for other domains
   # TODO: most inputs should be trimmed before validatation, probably some global logic?
@@ -627,7 +628,7 @@ class Domain < ApplicationRecord
   def as_json(_options)
     hash = super
     hash['auth_info'] = hash.delete('transfer_code') # API v1 requirement
-    hash['valid_from'] = hash['registered_at'] # API v1 requirement
+    hash['valid_from'] = hash['created_at'] # API v1 requirement
     hash.delete('statuses_before_force_delete')
     hash
   end

app/models/epp/domain.rb

@@ -41,7 +41,6 @@ class Epp::Domain < Domain
       domain = Epp::Domain.new
       domain.attributes = domain.attrs_from(frame, current_user)
       domain.attach_default_contacts
-      domain.registered_at = Time.zone.now
 
       period = domain.period.to_i
       plural_period_unit_name = (domain.period_unit == 'm' ? 'months' : 'years').to_sym
@@ -150,7 +149,6 @@ class Epp::Domain < Domain
 
     at[:name] = frame.css('name').text if new_record?
     at[:registrar_id] = current_user.registrar.try(:id)
-    at[:registered_at] = Time.zone.now if new_record?
 
     period = frame.css('period').text
     at[:period] = (period.to_i == 0) ? 1 : period.to_i