Fix various issues with domain block import (#1944)

- stop using Paperclip for processing domain allow/block imports
- stop leaving temporary files
- better error handling
- assume CSV files are UTF-8-encoded

app/validators/admin_import_validator.rb

@@ -1,19 +0,0 @@
-# frozen_string_literal: true
-
-class AdminImportValidator < ActiveModel::Validator
-  FIRST_HEADER = '#domain'
-
-  def validate(import)
-    return if import.type.blank? || import.data.blank?
-
-    # We parse because newlines could be part of individual rows. This
-    # runs on create so we should be reading the local file here before
-    # it is uploaded to object storage or moved anywhere...
-    csv_data = CSV.parse(import.data.queued_for_write[:original].read)
-
-    row_count  = csv_data.size
-    row_count -= 1 if csv_data.first&.first == FIRST_HEADER
-
-    import.errors.add(:data, I18n.t('imports.errors.over_rows_processing_limit', count: Admin::DomainBlocksController::ROWS_PROCESSING_LIMIT)) if row_count > Admin::DomainBlocksController::ROWS_PROCESSING_LIMIT
-  end
-end