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.go

@@ -92,7 +92,7 @@ func (Settings) TableName() string { return "golem15_sitemap_settings" }
 
 // Fillable lists the columns the settings form edits.
 func (Settings) Fillable() []string {
-	return []string{"site_url", "file_size", "extra_urls"}
+	return []string{"site_url", "file_size", "extra_urls", "images_enabled", "news_enabled", "news_publication", "news_sources"}
 }
 
 // Rules is empty: the settings controller runs Validate before a save.
@@ -103,8 +103,13 @@ const (
 	errFileSize  = "golem15.sitemap::lang.errors.file_size"
 	errURLScheme = "golem15.sitemap::lang.errors.url_scheme"
 	errLastmod   = "golem15.sitemap::lang.errors.lastmod"
+	errNewsName  = "golem15.sitemap::lang.errors.news_publication"
+	errNewsSrc   = "golem15.sitemap::lang.errors.news_source"
 )
 
+// MaxNewsPublication is the longest publication name.
+const MaxNewsPublication = 255
+
 // Validate answers every problem Problems finds as a validation error on
 // its field; the settings controller calls it before a save, so the save
 // answers 422 and the row is left as it was.
@@ -140,9 +145,52 @@ func (s Settings) Problems() map[string][]string {
 			out["extra_urls."+strconv.Itoa(i)+".lastmod"] = []string{errLastmod}
 		}
 	}
+	if name := Text(s.NewsPublication); utf8.RuneCountInString(name) > MaxNewsPublication || strings.ContainsFunc(name, unicode.IsControl) {
+		out["news_publication"] = []string{errNewsName}
+	}
+	for _, v := range s.NewsSources.Get() {
+		if !ValidNewsSource(strings.TrimSpace(v)) {
+			out["news_sources"] = []string{errNewsSrc}
+			break
+		}
+	}
 	return out
 }
 
+// The prefixes of the news_sources values (P33-15): type:<type key> and
+// category:<type key>:<id>.
+const (
+	NewsSourceType     = "type:"
+	NewsSourceCategory = "category:"
+)
+
+// ValidNewsSource reports whether v is a news_sources value:
+// type:<type key> or category:<type key>:<id>, each part 1 to 128 letters,
+// digits, '-', '_' or '.'.
+func ValidNewsSource(v string) bool {
+	switch {
+	case strings.HasPrefix(v, NewsSourceType):
+		return validSourcePart(strings.TrimPrefix(v, NewsSourceType))
+	case strings.HasPrefix(v, NewsSourceCategory):
+		rest := strings.TrimPrefix(v, NewsSourceCategory)
+		i := strings.LastIndex(rest, ":")
+		return i > 0 && validSourcePart(rest[:i]) && validSourcePart(rest[i+1:])
+	}
+	return false
+}
+
+func validSourcePart(s string) bool {
+	if s == "" || len(s) > 128 {
+		return false
+	}
+	for _, r := range s {
+		if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9' || r == '-' || r == '_' || r == '.') {
+			return false
+		}
+	}
+	return true
+}
+
 // AllowedFrontendURL reports whether u may be the site address: the empty
 // string, or an absolute http or https URL with a host and without
 // whitespace, control characters, a double quote, <, > or &. The value

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