feat: HTML sitemap groups API

- GET /_sitemap/api/v1/html?locale= answers the overview: the listed
  addresses of the XML sitemaps by group, in the order pages, offers,
  services, categories (as a tree), posts (newest first), tags, authors,
  other content types, each with its title in the locale, total, the
  first html_page_size links and more
- GET /_sitemap/api/v1/html/{group}?locale=&page= answers one page of a
  group; past the last page, an unknown group or the HTML sitemap off
  answer 404; every answer is noindex; per-IP bucket sitemap-html
- the HTML sitemap tab: html_enabled, the address per locale (html_paths)
  and html_page_size 50 to 500, with errors.html_path and errors.page_size
- contract.Listing.Settings answers HTMLEnabled, HTMLPaths, HTMLPageSize
  and KeyFile

models/settings/fields.yaml

@@ -127,3 +127,21 @@ fields:
             action: hide
             field: indexnow_enabled
             condition: unchecked
+
+    # The address per locale (html_path, an mltext field) is added after
+    # this switch by the plugin's form extension, stored in html_paths.
+    html_enabled:
+        label: golem15.sitemap::lang.settings.html_enabled
+        comment: golem15.sitemap::lang.settings.html_enabled_comment
+        type: switch
+        default: true
+        span: full
+        tab: golem15.sitemap::lang.settings.tab_html
+
+    html_page_size:
+        label: golem15.sitemap::lang.settings.html_page_size
+        comment: golem15.sitemap::lang.settings.html_page_size_comment
+        type: number
+        default: 150
+        span: right
+        tab: golem15.sitemap::lang.settings.tab_html