Build breadcrumbs from the conventions rather than from code

Problem/Motivation

Breadcrumb trails are currently literal arrays inside each resource — the article's trail names the blog page by its node id. It is the second clearest case, after the special pages, of one site's decisions living in a module meant for any site.

Proposed resolution

• A breadcrumb service that takes a trail of page roles plus self from the convention and resolves each to a title and an alias in the response language.
• A role that is unfilled is skipped, shortening the trail rather than producing a hole.
• Hierarchical trails for terms, where the parent chain is the trail.
• The same service used by every resource kind, so a trail looks the same wherever it appears.

Remaining tasks

Everything.