Move data migrations from rake tasks to gem

See #1298

lib/tasks/data_migrations/convert_domain_delete_date.rake

@@ -1,16 +0,0 @@
-namespace :data_migrations do
-  task convert_domain_delete_date: :environment do
-    processed_domain_count = 0
-
-    Domain.transaction do
-      Domain.find_each do |domain|
-        next unless domain.delete_date
-
-        domain.update_columns(delete_date: domain.delete_date + 1.day)
-        processed_domain_count += 1
-      end
-    end
-
-    puts "Domains processed: #{processed_domain_count}"
-  end
-end

lib/tasks/data_migrations/delete_orphaned_registrant_verifications.rake

@@ -1,15 +0,0 @@
-namespace :data_migrations do
-  task delete_orphaned_registrant_verifications: :environment do
-    orphaned_registrant_verifications = RegistrantVerification.where.not(domain_id: Domain.ids)
-    orphaned_registrant_verification_count = orphaned_registrant_verifications.count
-    processed_registrant_verification_count = 0
-
-    orphaned_registrant_verifications.each do |registrant_verification|
-      registrant_verification.destroy!
-      processed_registrant_verification_count += 1
-    end
-
-    puts "Processed: #{processed_registrant_verification_count} out of" \
-      " #{orphaned_registrant_verification_count}"
-  end
-end

lib/tasks/data_migrations/regenerate_registrar_reference_numbers.rake

@@ -1,16 +0,0 @@
-namespace :data_migrations do
-  task regenerate_registrar_reference_numbers: [:environment] do
-    processed_registrar_count = 0
-
-    Registrar.transaction do
-      Registrar.all.each do |registrar|
-        next unless registrar.reference_no.start_with?('RF')
-
-        registrar.update_columns(reference_no: Billing::ReferenceNo.generate)
-        processed_registrar_count += 1
-      end
-    end
-
-    puts "Registrars processed: #{processed_registrar_count}"
-  end
-end