feat: Google News sitemap

- sitemap-news.xml lists the articles (content kind article) first
  published in the last two days, newest first, at most 1000, with the
  publication name (news_publication, else app.name), the language of the
  locale, the publication date and the title
- news_sources narrows it to type:<key> and category:<key>:<id> values;
  the Images and News tab lists the values to use in a partial
- the index lists the file while news_enabled is on; otherwise it is 404

models/settings/fields.yaml

@@ -48,3 +48,54 @@ fields:
                     comment: golem15.sitemap::lang.settings.extra_url_lastmod_comment
                     type: text
                     span: right
+
+    images_enabled:
+        label: golem15.sitemap::lang.settings.images_enabled
+        comment: golem15.sitemap::lang.settings.images_enabled_comment
+        type: switch
+        default: true
+        span: full
+        tab: golem15.sitemap::lang.settings.tab_media
+
+    news_enabled:
+        label: golem15.sitemap::lang.settings.news_enabled
+        comment: golem15.sitemap::lang.settings.news_enabled_comment
+        type: switch
+        default: false
+        span: full
+        tab: golem15.sitemap::lang.settings.tab_media
+
+    news_publication:
+        label: golem15.sitemap::lang.settings.news_publication
+        comment: golem15.sitemap::lang.settings.news_publication_comment
+        type: text
+        span: left
+        tab: golem15.sitemap::lang.settings.tab_media
+        trigger:
+            action: hide
+            field: news_enabled
+            condition: unchecked
+
+    # Taglist options are a fixed list, so the article types and their
+    # categories are listed by the news_sources partial below instead.
+    news_sources:
+        label: golem15.sitemap::lang.settings.news_sources
+        comment: golem15.sitemap::lang.settings.news_sources_comment
+        type: taglist
+        mode: array
+        span: right
+        tab: golem15.sitemap::lang.settings.tab_media
+        trigger:
+            action: hide
+            field: news_enabled
+            condition: unchecked
+
+    _news_sources:
+        type: partial
+        path: news_sources
+        span: full
+        tab: golem15.sitemap::lang.settings.tab_media
+        trigger:
+            action: hide
+            field: news_enabled
+            condition: unchecked