Ring graph for 1-month freshness data.

[mshenfield]

Include a simple ring graph at bottom or on mobile for overall data freshness. This may help with issue #10 on the responsive layout. We could have the percentage of data that is 0-1 month fresh in the center, and each chunk of the ring be a different category (0-1 month, 1-6months, 6-12 months, 1 - 2 years, 2+ years) of freshness.

Reference screenshot (left view) of ring graph.