feat(17-03): full sitemap generation, settings, hooks and import commands

- generator in Definition::addItemToSet order: 50000 URL cap, lastmod from
  the record or the updates manifest (regular file, 5 MiB cap, fail safe),
  BeforeAddItem (skip) and AddItem (extra XML) festival events, tier or item
  priority, per-alternate expansion of list type children
- priority tiers from golem15.sitemap.priority_tiers, compiled at boot,
  defaults equal to the PHP getPriorityForUrl rules
- frontend_url rewrite of loc, xhtml:link href and the stylesheet href;
  GET /sitemap.xsl serves the PHP stylesheet byte for byte
- golem15_sitemap_settings singleton and Settings model
- sitemap:import (PHP ImportCommand) and sitemap:import-winter with the
  read-only Winter export SQL
- urlto and full goldens generated from PHP; TestSitemapImportSmoke

scripts/winter-export-definitions.sql

@@ -0,0 +1,27 @@
+-- Sitemap definitions of a WinterCMS site with the Golem15.Sitemap plugin,
+-- one JSON object per line: the --definitions input of
+-- sitemap:import-winter.
+--
+--   mysql -N -B --raw <database> < winter-export-definitions.sql > sitemap-definitions.jsonl
+--
+-- Read-only: it selects from the definitions table and changes nothing.
+-- JSON_OBJECT needs MySQL 5.7.8+ or MariaDB 10.2.3+.
+--
+-- Check the table name on the server before running:
+--   SHOW TABLES LIKE '%sitemap_definitions';
+-- This file assumes golem15_sitemap_definitions. A site still on the
+-- Winter.Sitemap or RainLab.Sitemap plugin uses winter_sitemap_definitions
+-- or rainlab_sitemap_definitions; change the FROM line to match.
+--
+-- data is the stored item list as text; the command decodes it. A site with
+-- several themes exports one row per theme; pass --theme to the command to
+-- pick the one to copy.
+SELECT JSON_OBJECT(
+    'id', d.id,
+    'theme', d.theme,
+    'data', d.data,
+    'created_at', d.created_at,
+    'updated_at', d.updated_at
+)
+FROM golem15_sitemap_definitions d
+ORDER BY d.id;