Cleanup deprecated HtmlAttachment attributes

This completes the work that extracted govspeak content out of `HtmlAttachment` and into a separate `GovspeakContent` model.

* [x] [Main story](https://www.agileplannerapp.com/boards/173808/cards/6159) successfully deployed to production
* [ ] Code review
* [ ] PO accepted

Story: https://www.agileplannerapp.com/boards/173808/cards/6223
don't we need to anonymise `govspeak_content_body` instead?