Docx conversion

REFERENCES
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https://www.pivotaltracker.com/story/show/81549072

This pull request completes the conversion to DOCX

OUTCOME DESIRED
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We'd like to export to DOCX in Tahi so users can keep working in Microsoft Word should they desire or if publishers/syndicators downstream require the format.

ACCEPTANCE CRITERIA
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You should see DOCX as an option in the Downloads dropdown. Clicking on it should download a DOCX file. 
Do we need a legit IHAT_URL for tests to work? There's a VCR cassette checked in, so I'm thinking the URL is arbitrary.

No biggie either way.