🐛 Clear Document.pdf_file after FoiAttachment redaction

[pajowu]

If the pdf_file is set (for example because the document pdf needed to be rewritten and get_writeable_file was called), it will take precedence over the attachments pdf. This means that new redactions applied to the attachment after pdf_file is set are not shown in the document, as the old version from pdf_file is still used for generating the pages etc.

[stefanw]

I do not recall when I introduced get_writeable_file. I thought the concept of original and pdf_file in filingcabinet where mutually exclusive. I thought if there's an original like an attachment, there would be no other stored document. I guess there can be edge cases where you do need one, but I'm a bit concerned that there's logic in filingcabinet that will ignore pdf_file when original is still set – or that when it's not set but there should be a related model. Obviously, that is nothing that can be fixed in froide. If this signal processor helps, let's go!