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/editor_error.go

@@ -27,13 +27,13 @@ func editorHandleFileOperationErrorRender(ctx *context_service.Context, message,
 	flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 		"Message": message,
 		"Summary": summary,
-		"Details": utils.SanitizeFlashErrorString(details),
+		"Details": utils.EscapeFlashErrorString(details),
 	})
 	if err == nil {
 		ctx.JSONError(flashError)
 	} else {
-		log.Error("RenderToHTML: %v", err)
-		ctx.JSONError(message + "\n" + summary + "\n" + utils.SanitizeFlashErrorString(details))
+		log.Error("RenderToHTML(%q, %q, %q), error: %v", message, summary, details, err)
+		ctx.JSONError("Unable to render error details, see server logs") // it should never happen
 	}
 }