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/locales/en.yml

@@ -421,6 +421,8 @@ en:
       add_new: Allow federation with domain
       created_msg: Domain has been successfully allowed for federation
       destroyed_msg: Domain has been disallowed from federation
+      export: Export
+      import: Import
       undo: Disallow federation with domain
     domain_blocks:
       add_new: Add new domain block
@@ -429,6 +431,8 @@ en:
       domain: Domain
       edit: Edit domain block
       existing_domain_block_html: You have already imposed stricter limits on %{name}, you need to <a href="%{unblock_url}">unblock it</a> first.
+      export: Export
+      import: Import
       new:
         create: Create block
         hint: The domain block will not prevent creation of account entries in the database, but will retroactively and automatically apply specific moderation methods on those accounts.
@@ -469,6 +473,14 @@ en:
       resolved_dns_records_hint_html: The domain name resolves to the following MX domains, which are ultimately responsible for accepting e-mail. Blocking an MX domain will block sign-ups from any e-mail address which uses the same MX domain, even if the visible domain name is different. <strong>Be careful not to block major e-mail providers.</strong>
       resolved_through_html: Resolved through %{domain}
       title: Blocked e-mail domains
+    export_domain_allows:
+      new:
+        title: Import domain allows
+      no_file: No file selected
+    export_domain_blocks:
+      new:
+        title: Import domain blocks
+      no_file: No file selected
     follow_recommendations:
       description_html: "<strong>Follow recommendations help new users quickly find interesting content</strong>. When a user has not interacted with others enough to form personalized follow recommendations, these accounts are recommended instead. They are re-calculated on a daily basis from a mix of accounts with the highest recent engagements and highest local follower counts for a given language."
       language: For language

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