Fix Plainify edge cases

This commit replaces the main part of `helpers.StripHTML` with Go's implementation in its html/template package.

It's a little slower, but correctness is more important:

```bash
BenchmarkStripHTMLOld-10    	  680316	      1764 ns/op	     728 B/op	       4 allocs/op
BenchmarkStripHTMLNew-10    	  384520	      3099 ns/op	    2089 B/op	      10 allocs/op
```

Fixes #9199
Fixes #9909
Closes #9410

tpl/internal/go_templates/htmltemplate/hugo_template.go

@@ -34,3 +34,8 @@ func (t *Template) Prepare() (*template.Template, error) {
 	}
 	return t.text, nil
 }
+
+// See https://github.com/golang/go/issues/5884
+func StripTags(html string) string {
+	return stripTags(html)
+}