Use markdown frontmatter to provide Table of contents, language and frontmatter rendering (#11047)

* Add control for the rendering of the frontmatter
* Add control to include a TOC
* Add control to set language - allows control of ToC header and CJK glyph choice.

Signed-off-by: Andrew Thornton art27@cantab.net

modules/markup/markdown/toc.go

@@ -0,0 +1,49 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package markdown
+
+import (
+	"fmt"
+	"net/url"
+
+	"github.com/unknwon/i18n"
+	"github.com/yuin/goldmark/ast"
+)
+
+func createTOCNode(toc []Header, lang string) ast.Node {
+	details := NewDetails()
+	summary := NewSummary()
+
+	summary.AppendChild(summary, ast.NewString([]byte(i18n.Tr(lang, "toc"))))
+	details.AppendChild(details, summary)
+	ul := ast.NewList('-')
+	details.AppendChild(details, ul)
+	currentLevel := 6
+	for _, header := range toc {
+		if header.Level < currentLevel {
+			currentLevel = header.Level
+		}
+	}
+	for _, header := range toc {
+		for currentLevel > header.Level {
+			ul = ul.Parent().(*ast.List)
+			currentLevel--
+		}
+		for currentLevel < header.Level {
+			newL := ast.NewList('-')
+			ul.AppendChild(ul, newL)
+			currentLevel++
+			ul = newL
+		}
+		li := ast.NewListItem(currentLevel * 2)
+		a := ast.NewLink()
+		a.Destination = []byte(fmt.Sprintf("#%s", url.PathEscape(header.ID)))
+		a.AppendChild(a, ast.NewString([]byte(header.Text)))
+		li.AppendChild(li, a)
+		ul.AppendChild(ul, li)
+	}
+
+	return details
+}