D3.js v3 -> v4. Switch out our D3.js basecode for v3 D3 code that's better

[thundergolfer]

4 main things

• This changes from D3 v4 to D3 v3
• It swaps out the old v4 demo code for new d3 demo code that has more features
• It adds a new library for doing activity heatmaps (like is on Github profiles)
• It removes the "coming soon stuff" from the homepage

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We really need to sort out our layout and CSS.

[thundergolfer]