Refactor some Str2html code (#29397)

This PR touches the most interesting part of the "template refactoring".

1. Unclear variable type. Especially for "web/feed/convert.go":
sometimes it uses text, sometimes it uses HTML.
2. Assign text content to "RenderedContent" field, for example: `
project.RenderedContent = project.Description` in web/org/projects.go
3. Assign rendered content to text field, for example: `r.Note =
rendered content` in web/repo/release.go
4. (possible) Incorrectly calling `{{Str2html
.PackageDescriptor.Metadata.ReleaseNotes}}` in
package/content/nuget.tmpl, I guess the name Str2html misleads
developers to use it to "render string to html", but it only sanitizes.
if ReleaseNotes really contains HTML, then this is not a problem.

modules/markup/markdown/markdown.go

@@ -6,6 +6,7 @@ package markdown
 
 import (
 	"fmt"
+	"html/template"
 	"io"
 	"strings"
 	"sync"
@@ -262,12 +263,12 @@ func Render(ctx *markup.RenderContext, input io.Reader, output io.Writer) error
 }
 
 // RenderString renders Markdown string to HTML with all specific handling stuff and return string
-func RenderString(ctx *markup.RenderContext, content string) (string, error) {
+func RenderString(ctx *markup.RenderContext, content string) (template.HTML, error) {
 	var buf strings.Builder
 	if err := Render(ctx, strings.NewReader(content), &buf); err != nil {
 		return "", err
 	}
-	return buf.String(), nil
+	return template.HTML(buf.String()), nil
 }
 
 // RenderRaw renders Markdown to HTML without handling special links.