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/home.tmpl

@@ -6,7 +6,7 @@
 		<div class="ui mobile reversed stackable grid">
 			<div class="ui {{if .ShowMemberAndTeamTab}}eleven wide{{end}} column">
 				{{if .ProfileReadme}}
-					<div id="readme_profile" class="markup">{{.ProfileReadme | Str2html}}</div>
+					<div id="readme_profile" class="markup">{{.ProfileReadme}}</div>
 				{{end}}
 				{{template "explore/repo_search" .}}
 				{{template "explore/repo_list" .}}