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/instances/index.html.haml

@@ -4,8 +4,12 @@
 - content_for :heading_actions do
   - if whitelist_mode?
     = link_to t('admin.domain_allows.add_new'), new_admin_domain_allow_path, class: 'button', id: 'add-instance-button'
+    = link_to t('admin.domain_allows.export'), export_admin_export_domain_allows_path(format: :csv), class: 'button'
+    = link_to t('admin.domain_allows.import'), new_admin_export_domain_allow_path, class: 'button'
   - else
     = link_to t('admin.domain_blocks.add_new'), new_admin_domain_block_path, class: 'button', id: 'add-instance-button'
+    = link_to t('admin.domain_blocks.export'), export_admin_export_domain_blocks_path(format: :csv), class: 'button'
+    = link_to t('admin.domain_blocks.import'), new_admin_export_domain_block_path, class: 'button'
 
 .filters
   .filter-subset