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/templates/util_string.go

@@ -4,6 +4,8 @@
 package templates
 
 import (
+	"fmt"
+	"html/template"
 	"strings"
 
 	"code.gitea.io/gitea/modules/base"
@@ -17,6 +19,19 @@ func NewStringUtils() *StringUtils {
 	return &stringUtils
 }
 
+func (su *StringUtils) ToString(v any) string {
+	switch v := v.(type) {
+	case string:
+		return v
+	case template.HTML:
+		return string(v)
+	case fmt.Stringer:
+		return v.String()
+	default:
+		return fmt.Sprint(v)
+	}
+}
+
 func (su *StringUtils) HasPrefix(s, prefix string) bool {
 	return strings.HasPrefix(s, prefix)
 }