site: progress bar on every milestone

Every milestone page and every card of the milestones list shows a stacked bar of its issues: done, in review, waiting, with the totals as heading and a legend. Computed from the milestone data by the template, no JavaScript. The line of counts in the Next box and the "N of M closed" on the cards are replaced by it.

Verified locally with a hugo build and headless Chrome screenshots in light and dark.

Closes #11 (closed)