Handle cases when LibreOffice hangs

[apyrgio]

When running Dangerzone against our large test set, we found that some files (e.g., fdo78883.docx and ofz21168-1.doc) make LibreOffice 7.6 hang.

We opened a bug report for these files, but until the underlying issue is solved, we need a way to detect such hangs, and stop the conversion.

[apyrgio]

Re-introducing timeouts for the whole document is a solution I'd personally like to avoid. They have bitten us a lot in the past (#749), they are arbitrary (documents with many pages lead to very large timeout times), and we have recently decided to ditch them altogether (#687).

What makes more sense to me is the following:

• Get the number of pages in the document. Here's a way to do so: https://askubuntu.com/questions/305633/how-can-i-determine-the-page-count-of-odt-doc-docx-and-other-office-documents
• Spin up an UNO server (https://github.com/unoconv/unoserver). This server is responsible for loading a document, and listening for API requests.
• Send API requests to the UNO server, and ask to convert the document a single page at a time.
  • See some supported export parameters that LibreOffice offers by default, and the blog post by the LibreOffice dev that added those. The PageRange option is of interest to us.
  • The UNO server project provides a command-line client (unoconvert), but maybe we can send these API requests programmatically.
• Set a timeout for each API request. Since at the API request level we know we're dealing with a signle document page, we can set a sensible timeout (e.g., 3 minutes). Anecdotally, converting a .docx of ~2000 pages took in my laptop 18 minutes, so this timeout is more than reasonable.

Some extra benefits of this approach:

• UNO server has an option to send files back as binary data, instead of writing them to the filesystem. This will help with Defense in Depth - Traceless Sanitization #633.
• We aim to introduce file previewing in Dangerzone (see File Preview PoC #758 for a PoC). One concern we have with file previewing is that LibreOffice documents may take a while to be converted to PDFs, so that we can stream their pixels afterwards. With this method, we can start streaming from the very first page.
• Compared with providing PageRange arguments via the LibreOffice CLI, UNO server loads the document in memory once, so it offers faster conversion times for documents with lots of pages.

[eloquence]

(Leaving unmilestoned for now given lower potential impact)