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-settings.sql

@@ -0,0 +1,16 @@
+-- Sitemap settings of a WinterCMS site with the Golem15.Sitemap plugin: the
+-- JSON object the plugin keeps in system_settings, the --settings input of
+-- sitemap:import-winter.
+--
+--   mysql -N -B --raw <database> < winter-export-settings.sql > sitemap-settings.json
+--
+-- Read-only: it selects one system_settings value and changes nothing.
+--
+-- Check that the row exists before running:
+--   SELECT item FROM system_settings WHERE item LIKE '%sitemap%';
+-- An empty result means the settings were never saved; skip --settings and
+-- the command keeps the defaults.
+SELECT s.value
+FROM system_settings s
+WHERE s.item = 'golem15_sitemap_settings'
+LIMIT 1;