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.

templates/org/header.tmpl

@@ -18,7 +18,7 @@
 				{{end}}
 			</span>
 		</div>
-		{{if .RenderedDescription}}<div class="render-content markup">{{.RenderedDescription | Str2html}}</div>{{end}}
+		{{if .RenderedDescription}}<div class="render-content markup">{{.RenderedDescription}}</div>{{end}}
 		<div class="text light meta gt-mt-2">
 			{{if .Org.Location}}<div class="flex-text-block">{{svg "octicon-location"}} <span>{{.Org.Location}}</span></div>{{end}}
 			{{if .Org.Website}}<div class="flex-text-block">{{svg "octicon-link"}} <a class="muted" target="_blank" rel="noopener noreferrer me" href="{{.Org.Website}}">{{.Org.Website}}</a></div>{{end}}