Optimise 'republish_non_english_html_attachments' for bulk publish
Optimise 'republish_non_english_html_attachments' for bulk publish

Without this, running the rake task would trigger ~10k items for
the Sidekiq queue and would process around 2.5k of them before
stagnating. Looking at the Sidekiq web monitoring, we can see
the workers all begin to fail with the same error:

```
ActiveRecord::LockWaitTimeout: Mysql2::Error::TimeoutError: Lock wait timeout exceeded; try restarting transaction
```

We must have come across this problem before. Whilst we're
calling `perform_async`, which hands the workers off to a queue,
we've not been setting the `bulk_publishing` parameter to true
(it [defaults to false][]). I've taken inspiration from elsewhere
in the codebase ([1][]) to fix this up.

[1]: https://github.com/alphagov/whitehall/blob/508c1e9fe332966f1e962c07ad6238590962057e/app/models/corporate_information_page.rb
[defaults to false]: https://github.com/alphagov/whitehall/blob/cd718bf2d90aa198767e8d4328d3f3fd1ef13e02/app/workers/publishing_api_document_republishing_worker.rb#L21

Trello: https://trello.com/c/3BeDlugo/2761-republish-whitehall-documents-containing-non-english-html-attachments-5

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall