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/repo/release/list.tmpl

@@ -58,7 +58,7 @@
 							{{end}}
 						</p>
 						<div class="markup desc">
-							{{Str2html $release.Note}}
+							{{$release.RenderedNote}}
 						</div>
 						<div class="divider"></div>
 						<details class="download" {{if eq $idx 0}}open{{end}}>