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

routers/web/repo/issue_new.go

@@ -170,7 +170,7 @@ func renderErrorOfTemplates(ctx *context.Context, errs map[string]error) templat
 	flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 		"Message": ctx.Tr("repo.issues.choose.ignore_invalid_templates"),
 		"Summary": ctx.Tr("repo.issues.choose.invalid_templates", len(errs)),
-		"Details": utils.SanitizeFlashErrorString(strings.Join(lines, "\n")),
+		"Details": utils.EscapeFlashErrorString(strings.Join(lines, "\n")),
 	})
 	if err != nil {
 		log.Debug("render flash error: %v", err)