archive page: show origin breakdown

It would be nice to show on the archive page, where we show our main sources of ingested source code, a breakdown of how many origins come from each source.

Rationale: historically GitHub has been the dominating source, quantity-wise; and that's still the case. But origins like GitLab.com are growing fast and it would be nice to highlight that.

Concrete UI proposal:

1. a pie chart on the size of the current logo-flow, showing how many origins come from each source; and/or
2. the number of origins of each type typeset below each logo

(I'm unconvinced about (2), it can clutter the screen quite a bit, but it's a worth a try.)

Hat tip to @rdicosmo for the idea.

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Migrated from T1463 (view on Phabricator)