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/views/admin/export_domain_allows/new.html.haml

@@ -0,0 +1,10 @@
+- content_for :page_title do
+  = t('.title')
+
+= simple_form_for @import, url: import_admin_export_domain_allows_path, html: { multipart: true } do |f|
+  .fields-row
+    .fields-group.fields-row__column.fields-row__column-6
+      = f.input :data, wrapper: :with_block_label, hint: t('simple_form.hints.imports.data'), as: :file
+
+  .actions
+    = f.button :button, t('imports.upload'), type: :submit