Make `data-visualization` work offline

[benoit74]

This is the overall goal of the milestone. It hence serves both as a basic requirement, instruction on what to look after during development and final checkup.

The goal of this milestone is to generate data-visualization ZIMs (one per supported languages). In FCC repo, all challenges markdown (in English) are stored under 04-data-visualization folder.

The command to create the EN ZIM is going to look like (title, description still have to be determined):

fcc2zim --course=data-visualization-with-d3,json-apis-and-ajax --name tests_en_freecodecamp-04-data-visualization --zim-file tests_en_freecodecamp-04-data-visualization.zim --title "freeCodeCamp 04" --description "Data Visualization" --language eng --overwrite

Note that data-visualization-projects course has been intentionally not considered for inclusion inside the ZIM because it relies on CodePen for evaluation.

Final checklist

• Confirm list of audited spoken languages
• ZIM can be generated for all these spoken languages
• all challenges can be solved
• nothing is broken in the UI
• no external resources are significantly missing (sample project for an exercise, image, video, ...)