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/base/alert.tmpl

@@ -1,20 +1,20 @@
 {{if .Flash.ErrorMsg}}
 	<div class="ui negative message flash-message flash-error">
-		<p>{{.Flash.ErrorMsg | Str2html}}</p>
+		<p>{{.Flash.ErrorMsg | SanitizeHTML}}</p>
 	</div>
 {{end}}
 {{if .Flash.SuccessMsg}}
 	<div class="ui positive message flash-message flash-success">
-		<p>{{.Flash.SuccessMsg | Str2html}}</p>
+		<p>{{.Flash.SuccessMsg | SanitizeHTML}}</p>
 	</div>
 {{end}}
 {{if .Flash.InfoMsg}}
 	<div class="ui info message flash-message flash-info">
-		<p>{{.Flash.InfoMsg | Str2html}}</p>
+		<p>{{.Flash.InfoMsg | SanitizeHTML}}</p>
 	</div>
 {{end}}
 {{if .Flash.WarningMsg}}
 	<div class="ui warning message flash-message flash-warning">
-		<p>{{.Flash.WarningMsg | Str2html}}</p>
+		<p>{{.Flash.WarningMsg | SanitizeHTML}}</p>
 	</div>
 {{end}}

templates/base/alert_details.tmpl

@@ -2,6 +2,6 @@
 <details>
 	<summary>{{.Summary}}</summary>
 	<code>
-		{{.Details | Str2html}}
+		{{.Details | SanitizeHTML}}
 	</code>
 </details>