Allow import/export of instance-level domain blocks/allows (#1754)

* Allow import/export of instance-level domain blocks/allows.
Fixes #15095

* Pacify circleci

* Address simple code review feedback

* Add headers to exported CSV

* Extract common import/export functionality to
AdminExportControllerConcern

* Add additional fields to instance-blocked domain export

* Address review feedback

* Split instance domain block/allow import/export into separate pages/controllers

* Address code review feedback

* Pacify DeepSource

* Work around Paperclip::HasAttachmentFile for Rails 6

* Fix deprecated API warning in export tests

* Remove after_commit workaround

app/models/domain_allow.rb

@@ -23,6 +23,10 @@ class DomainAllow < ApplicationRecord
       !rule_for(domain).nil?
     end
 
+    def allowed_domains
+      select(:domain)
+    end
+
     def rule_for(domain)
       return if domain.blank?