Phase 2: chapter nodes, aggregate partial, fixes

- _chapter.html.erb: new partial for erfa/chaostreff aggregated lists;
  renders title, location, external_url, sanitized body
- content_helper: fix aggregate attr regex to allow hyphens in values
  (erfa-detail tag was silently dropped); add debug logging (remove)
- page.rb: suppress libxml stderr noise in rewrite_links_in_body
- db/seeds/chapters.rb: one-shot seed script for erfa and chaostreff
  chapter nodes under parent nodes 548/549; creates bilingual pages,
  external_url, primary events with RRULEs where known
  Note: run Node.rebuild!(false) after execution to fix lft/rgt values

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