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/admin/import.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+# A non-activerecord helper class for csv upload
+class Admin::Import
+  extend ActiveModel::Callbacks
+  include ActiveModel::Model
+  include Paperclip::Glue
+
+  FILE_TYPES = %w(text/plain text/csv application/csv).freeze
+
+  # Paperclip required callbacks
+  define_model_callbacks :save, only: [:after]
+  define_model_callbacks :destroy, only: [:before, :after]
+
+  attr_accessor :data_file_name, :data_content_type
+
+  has_attached_file :data
+  validates_attachment_content_type :data, content_type: FILE_TYPES
+  validates_attachment_presence :data
+  validates_with AdminImportValidator, on: :create
+
+  def save
+    run_callbacks :save
+  end
+
+  def destroy
+    run_callbacks :destroy
+  end
+end