Added flat CSV downloading option for Census

[roll]

• fixes https://trello.com/c/fuO4zCd9/338-open-data-survey-africa-data-revolution-create-a-downloadable-results-file-where-all-records-of-the-survey-can-be-found-in-separ

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Flat CSV example:
entries.cascade.flat.csv.zip

[roll]

@dannylammerhirt
Please take a look (downloadable example in the first comment)

If the format in-general looks good I would suggest going thru column naming https://github.com/okfn/opendatasurvey/pull/1156/files#diff-72eef72e4a22ddeab37bd383e417cc58R314 to make it a little bit more consistent/human-friendly (I used the Google Doc but it's clearly a draft)

[roll]

@brew
Could you please review?

Also I think I need a hand to deploy these changes to http://test-african-data.survey.okfn.org/

[brew]

Looks good to me!

When you merge with master, and Travis has successfully run, a tagged release should be built on docker hub automatically (https://hub.docker.com/r/openknowledge/opendatasurvey/tags/). Use that tag as the release tag in our devops repo and do a make deploy... for the opendatasurvey as normal. This will update for all the surveys, not just test-africa-data.