Rename Str2html to SanitizeHTML and clarify its behavior (#29516)

Str2html was abused a lot. So use a proper name for it: SanitizeHTML

And add some tests to show its behavior.

templates/repo/settings/webhook/base_list.tmpl

@@ -10,7 +10,7 @@
 <div class="ui attached segment">
 	<div class="ui list">
 		<div class="item">
-			{{.Description | Str2html}}
+			{{.Description | SanitizeHTML}}
 		</div>
 		{{range .Webhooks}}
 			<div class="item truncated-item-container">