hugolib: Consider summary in front matter for .Summary

Add the ability to have a `summary` page variable that overrides
the auto-generated summary.  Logic for obtaining summary becomes:

  * if summary divider is present in content, use the text above it
  * if summary variables is present in page metadata, use that
  * auto-generate summary from first _x_ words of the content

Fixes #5800

tpl/transform/transform.go

@@ -15,7 +15,6 @@
 package transform
 
 import (
-	"bytes"
 	"html"
 	"html/template"
 
@@ -91,12 +90,6 @@ func (ns *Namespace) HTMLUnescape(s interface{}) (string, error) {
 	return html.UnescapeString(ss), nil
 }
 
-var (
-	markdownTrimPrefix         = []byte("<p>")
-	markdownTrimSuffix         = []byte("</p>\n")
-	markdownParagraphIndicator = []byte("<p")
-)
-
 // Markdownify renders a given input from Markdown to HTML.
 func (ns *Namespace) Markdownify(s interface{}) (template.HTML, error) {
 	ss, err := cast.ToStringE(s)
@@ -114,14 +107,9 @@ func (ns *Namespace) Markdownify(s interface{}) (template.HTML, error) {
 	)
 
 	// Strip if this is a short inline type of text.
-	first := bytes.Index(m, markdownParagraphIndicator)
-	last := bytes.LastIndex(m, markdownParagraphIndicator)
-	if first == last {
-		m = bytes.TrimPrefix(m, markdownTrimPrefix)
-		m = bytes.TrimSuffix(m, markdownTrimSuffix)
-	}
+	m = ns.deps.ContentSpec.TrimShortHTML(m)
 
-	return template.HTML(m), nil
+	return helpers.BytesToHTML(m), nil
 }
 
 // Plainify returns a copy of s with all HTML tags removed.