Refactor flash message and remove SanitizeHTML template func (#37179)

1. Fix the "flash message" layout problem for different cases
* I am sure most of the users should have ever seen the ugly
center-aligned error message with multiple lines.
2. Fix inconsistent "Details" flash message EOL handling, sometimes
`\n`, sometimes `<br>`
   * Now, always use "\n" and use `<pre>` to render
3. Remove SanitizeHTML template func because it is not useful and can be
easily abused.
* But it is still kept for mail templates, for example:
https://github.com/go-gitea/gitea/issues/36049
4. Clarify PostProcessCommitMessage's behavior and add FIXME comment

By the way: cleaned up some devtest pages, move embedded style block to
CSS file

templates/base/alert.tmpl

@@ -1,25 +1,9 @@
-{{- if .Flash.ErrorMsg -}}
-	<div class="ui negative message flash-message flash-error">
-		<p>{{.Flash.ErrorMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
-{{- if .Flash.SuccessMsg -}}
-	<div class="ui positive message flash-message flash-success">
-		<p>{{.Flash.SuccessMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
-{{- if .Flash.InfoMsg -}}
-	<div class="ui info message flash-message flash-info">
-		<p>{{.Flash.InfoMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
-{{- if .Flash.WarningMsg -}}
-	<div class="ui warning message flash-message flash-warning">
-		<p>{{.Flash.WarningMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
+{{- if .Flash.ErrorMsg}}{{ctx.RenderUtils.RenderFlashMessage "error" .Flash.ErrorMsg}}{{end -}}
+{{- if .Flash.WarningMsg}}{{ctx.RenderUtils.RenderFlashMessage "warning" .Flash.WarningMsg}}{{end -}}
+{{- if .Flash.InfoMsg}}{{ctx.RenderUtils.RenderFlashMessage "info" .Flash.InfoMsg}}{{end -}}
+{{- if .Flash.SuccessMsg}}{{ctx.RenderUtils.RenderFlashMessage "success" .Flash.SuccessMsg}}{{end -}}
 {{- if .ShowTwoFactorRequiredMessage -}}
-<div class="ui negative message flash-message flash-error">
-	<p><a href="{{AppSubUrl}}/user/settings/security/two_factor/enroll">{{ctx.Locale.Tr "auth.twofa_required"}}</a></p>
+<div class="ui error message flash-message flash-error">
+	<a href="{{AppSubUrl}}/user/settings/security/two_factor/enroll">{{ctx.Locale.Tr "auth.twofa_required"}}</a>
 </div>
 {{- end -}}

templates/base/alert_details.tmpl

@@ -2,10 +2,8 @@
 {{if .Details}}
 <details>
 	<summary>{{.Summary}}</summary>
-	{{.Details | SanitizeHTML}}
+	<pre>{{.Details}}</pre>
 </details>
 {{else}}
-<div>
-	{{.Summary}}
-</div>
+<div>{{.Summary}}</div>
 {{end}}