Basic docx generation

No custom formatting. Some presentational quirks, but it works. Custom formatting is non-trivial and isn't included here - seems we would need to create template files for the various possible combinations of formatting options. Perhaps we should decide on a limited set to make available to users. Same for header/footer. Formatting of exported Word documents is less crucial than for PDFs as user can (generally) edit after export. Need to hide export formatting options when user uses Word.