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

controllers/settings/_news_sources.htm

@@ -0,0 +1,19 @@
+{{- if .Data.Rows -}}
+<section>
+<h4>{{ trans "golem15.sitemap::lang.news.sources" }}</h4>
+<div class="summer-table-wrap">
+<table class="summer-table">
+<thead><tr><th>{{ trans "golem15.sitemap::lang.news.source_value" }}</th><th>{{ trans "golem15.sitemap::lang.news.source_label" }}</th></tr></thead>
+<tbody>
+{{- range .Data.Rows -}}
+<tr><td><code>{{ .Value }}</code></td><td>{{ .Label }}</td></tr>
+{{- end -}}
+</tbody>
+</table>
+</div>
+</section>
+{{- else -}}
+<div class="summer-callout" role="status">
+<p class="summer-callout__text">{{ trans "golem15.sitemap::lang.news.none" }}</p>
+</div>
+{{- end -}}