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. 
We were incorrectly naming the cassette. The cassette just downloads the valid JSON from api.github.com. Not ideal, but we didn't know what exactly what iHat would return at that point. I've added a chore to fix this.