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. 
I generally prefer using [Rack's symbols for status codes instead of integers](https://github.com/rack/rack/blob/master/lib/rack/utils.rb#L568) (ie: `:ok` for 200, `:not_found` for 404, etc). Particularly for status codes we don't come across often, such as 203. In this case it'd be `:non_authoritative_information`, which conveys more than 203.