feat: content descriptors for posts, categories and the new journal-tag type

- journal-post, journal-category and journal-tag carry pact.ContentDescriptor
  (controller, morph name, Describe, Records, Match) through signpost
- Match reads the locale prefix and the pattern with signpost.MatchPattern,
  finds the record by its slug in that locale, the default slug, another
  locale's slug or :id, and is exact only on the record's own URL (D-20c)
- journal-tag (/blog/tag/:slug, P-11) with references and Resolve; host key
  golem15.journal.tag_pages false empties tag URLs and stops tag matches
- TestContentDescriptors, TestJournalTagType; README documents both

lang/en/lang.yaml

@@ -137,6 +137,7 @@ menuitem:
   journal_post: Journal post
   all_journal_posts: All journal posts
   category_journal_posts: Journal category posts
+  journal_tag: Journal tag
 scopes:
   journal: Journal
   journal_description: Create, edit, publish and delete posts as you, within your Journal permissions.

lang/pl/lang.yaml

@@ -89,6 +89,7 @@ menuitem:
   journal_post: Wpis (Journal)
   all_journal_posts: Wszystkie wpisy (Journal)
   category_journal_posts: Wpisy kategorii (Journal)
+  journal_tag: Tag bloga
 scopes:
   journal: Journal
   journal_description: Twórz, edytuj, publikuj i usuwaj posty jako Ty, w ramach swoich uprawnień Journal.