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_render.go

@@ -208,7 +208,7 @@ func RenderMarkdownToHtml(ctx context.Context, input string) template.HTML { //n
 	if err != nil {
 		log.Error("RenderString: %v", err)
 	}
-	return template.HTML(output)
+	return output
 }
 
 func RenderLabels(ctx context.Context, labels []*issues_model.Label, repoLink string) template.HTML {