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/diff/comments.tmpl

@@ -55,7 +55,7 @@
 		<div class="ui attached segment comment-body">
 			<div class="render-content markup" {{if or $.Permission.IsAdmin $.HasIssuesOrPullsWritePermission (and $.root.IsSigned (eq $.root.SignedUserID .PosterID))}}data-can-edit="true"{{end}}>
 			{{if .RenderedContent}}
-				{{.RenderedContent|Str2html}}
+				{{.RenderedContent}}
 			{{else}}
 				<span class="no-content">{{ctx.Locale.Tr "repo.issues.no_content"}}</span>
 			{{end}}