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.

routers/web/repo/release.go

@@ -113,7 +113,7 @@ func getReleaseInfos(ctx *context.Context, opts *repo_model.FindReleasesOptions)
 			cacheUsers[r.PublisherID] = r.Publisher
 		}
 
-		r.Note, err = markdown.RenderString(&markup.RenderContext{
+		r.RenderedNote, err = markdown.RenderString(&markup.RenderContext{
 			Links: markup.Links{
 				Base: ctx.Repo.RepoLink,
 			},