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

config/routes.rb

@@ -194,7 +194,21 @@ Rails.application.routes.draw do
     get '/dashboard', to: 'dashboard#index'
 
     resources :domain_allows, only: [:new, :create, :show, :destroy]
-    resources :domain_blocks, only: [:new, :create, :destroy, :update, :edit]
+    resources :domain_blocks, only: [:new, :create, :show, :destroy, :update, :edit]
+
+    resources :export_domain_allows, only: [:new] do
+      collection do
+        get :export, constraints: { format: :csv }
+        post :import
+      end
+    end
+
+    resources :export_domain_blocks, only: [:new] do
+      collection do
+        get :export, constraints: { format: :csv }
+        post :import
+      end
+    end
 
     resources :email_domain_blocks, only: [:index, :new, :create] do
       collection do