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/mail/release.tmpl

@@ -22,7 +22,7 @@
 		{{.locale.Tr "mail.release.note"}}<br>
 		{{- if eq .Release.RenderedNote ""}}
 		{{else}}
-			{{.Release.RenderedNote | Str2html}}
+			{{.Release.RenderedNote}}
 		{{end -}}
 	</p>
 	<br><br>