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

config/locales/en.yml

@@ -1159,6 +1159,7 @@ en:
     invalid_markup: 'contains invalid HTML markup: %{error}'
   imports:
     errors:
+      invalid_csv_file: 'Invalid CSV file. Error: %{error}'
       over_rows_processing_limit: contains more than %{count} rows
     modes:
       merge: Merge