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/issue/view_content.tmpl

@@ -54,7 +54,7 @@
 					<div class="ui attached segment comment-body" role="article">
 						<div class="render-content markup" {{if or $.Permission.IsAdmin $.HasIssuesOrPullsWritePermission $.IsIssuePoster}}data-can-edit="true"{{end}}>
 							{{if .Issue.RenderedContent}}
-								{{.Issue.RenderedContent|Str2html}}
+								{{.Issue.RenderedContent}}
 							{{else}}
 								<span class="no-content">{{ctx.Locale.Tr "repo.issues.no_content"}}</span>
 							{{end}}