feat: H1 to H2 setting with the admin save conversion (tracer)

- content_h1_to_h2 and generic_anchors on the settings row (migration
  202610280001), Headings and links tab, config key
  golem15.journal.content_h1_to_h2 used when no settings row exists
- classes.ConvertH1 on the x/net/html tokenizer; the Blog Editor types
  convert while the setting is on and answer editor.h1_converted
- GET /posts/{slug} reports content_h1_to_h2 (additive key)

classes/blog_editor.go

@@ -14,6 +14,7 @@ import (
 	"unicode/utf8"
 
 	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/compass"
 	"git.golem15.com/golem15/summercms/modules/pact"
 	"gocloud.dev/blob"
 	xhtml "golang.org/x/net/html"
@@ -31,6 +32,9 @@ const (
 	editorRemovedAttributes = "golem15.journal::lang.editor.removed_attributes"
 	editorRemovedComments   = "golem15.journal::lang.editor.removed_comments"
 	editorMarkupChanged     = "golem15.journal::lang.editor.markup_changed"
+	// EditorH1Converted is the notice of a save that turned H1 headings
+	// into H2 (Count is the number of headings).
+	EditorH1Converted = "golem15.journal::lang.editor.h1_converted"
 )
 
 // blogListValueRunes is how many runes of text a list column shows.
@@ -79,6 +83,7 @@ var (
 // when unset).
 func BlogEditorTypes(app func() *backpack.App) []pact.FormFieldType {
 	upload := uploadBlogImage(app)
+	decode := decodeBlogHTMLFor(app)
 	return []pact.FormFieldType{
 		{
 			Name:         "blogeditor",
@@ -87,7 +92,7 @@ func BlogEditorTypes(app func() *backpack.App) []pact.FormFieldType {
 			Styles:       []string{"assets/dist/blogeditor.css"},
 			Options:      []string{"uploadFolder"},
 			CheckOptions: checkBlogOptions,
-			Decode:       decodeBlogHTML,
+			Decode:       decode,
 			Load:         loadBlogHTML,
 			ListValue:    blogListValue,
 			Upload:       upload,
@@ -100,7 +105,7 @@ func BlogEditorTypes(app func() *backpack.App) []pact.FormFieldType {
 			Styles:       []string{"assets/dist/blogeditor.css"},
 			Options:      []string{"htmlColumn", "uploadFolder"},
 			CheckOptions: checkBlogOptions,
-			Decode:       decodeBlogHTML,
+			Decode:       decode,
 			Load:         loadBlogHTML,
 			ListValue:    blogListValue,
 			Upload:       upload,
@@ -146,6 +151,47 @@ func checkUploadFolder(folder string) error {
 	return nil
 }
 
+// decodeBlogHTMLFor is the Blog Editor types' Decode: decodeBlogHTML, then,
+// while the H1 setting is on (EffectiveH1ToH2 on the save transaction and
+// the booted application's config), ConvertH1 on the stored HTML with the
+// EditorH1Converted notice counting the converted headings. app may be nil
+// or answer nil; the setting is then read from the database alone.
+func decodeBlogHTMLFor(app func() *backpack.App) func(context.Context, pact.FormFieldValue) (pact.FormFieldDecoded, error) {
+	return func(ctx context.Context, in pact.FormFieldValue) (pact.FormFieldDecoded, error) {
+		out, err := decodeBlogHTML(ctx, in)
+		if err != nil {
+			return out, err
+		}
+		text, isString := out.Value.(string)
+		if !isString || text == "" {
+			return out, nil
+		}
+		if !EffectiveH1ToH2(ctx, in.DB, appConfig(app)) {
+			return out, nil
+		}
+		converted, n := ConvertH1(text)
+		if n == 0 {
+			return out, nil
+		}
+		out.Value = converted
+		out.Notices = append(out.Notices, pact.FormFieldNotice{Message: EditorH1Converted, Count: n})
+		return out, nil
+	}
+}
+
+// appConfig is the config of the application app answers, nil when there
+// is none.
+func appConfig(app func() *backpack.App) *compass.Config {
+	if app == nil {
+		return nil
+	}
+	a := app()
+	if a == nil {
+		return nil
+	}
+	return a.Config
+}
+
 // decodeBlogHTML stores the HTML policy's output for a submitted value: the
 // exact submitted bytes with no notice when the policy keeps everything,
 // otherwise the stripped HTML with notices naming what was removed. nil