Add import/export support for word documents

[akprasad]

Some proofreaders like working with Word documents and are used to them. I think we can support this workflow with minimal changes to the larger system, so I want to try it out as an experiment.

Feature list

• In proofing/project.py, add a download_as_word_document function that supports downloading a proofing project as a Word document.
• In the same file, add an upload_word_document function that accepts a Word document and merges in any new and non-conflicting changes into the database. Since changing a project through an upload could be easily abused, this function should be annotated with the p2_required decorator, which restricts it to level 2 proofreaders.

Specification

• The document should encode which text belongs to which page and which version of the text was used, so that we know where text should be sent on upload. One way to do this is to wrap blocks of text with human-readable tags, such as <page n="5" version="3">text</page> to indicate that text is version 3 of page 5.