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.

(cherry picked from commit e71eb8930a5d0f60874b038c223498b41ad65592)

Conflicts:
	modules/templates/util_string.go
	trivial context conflict

routers/web/feed/profile.go

@@ -58,7 +58,7 @@ func showUserFeed(ctx *context.Context, formatType string) {
 	feed := &feeds.Feed{
 		Title:       ctx.Locale.TrString("home.feed_of", ctx.ContextUser.DisplayName()),
 		Link:        &feeds.Link{Href: ctx.ContextUser.HTMLURL()},
-		Description: ctxUserDescription,
+		Description: string(ctxUserDescription),
 		Created:     time.Now(),
 	}