web: directory crumbs on blob and blame pages link to the tree !187

crumbs built every component's URL with the page's own kind, so on a file page the parent directories linked to /blob/<ref>/<dir>, a 404. Only the leaf is a blob or blame page; the rest are directories and now link to the tree. TestCrumbsLinkDirectoriesToTree covers blob, blame, tree and the root.

Stacked on the font routes fix; merges onto main when that lands.

Closes #103