Improve "new-menu" (#24465)

I am not sure what "new-menu" means, but I think we need to fix these
problems:

1. it shouldn't have "stackable", which makes the items stacked when
width is small. the `new-menu` already has `overflow: auto`
2. `justify-content: center` doesn't work with `overflow: auto` (for
small width), so use `margin: auto`
*
https://bhch.github.io/posts/2021/04/centring-flex-items-and-allowing-overflow-scroll/
3. `runner-new-menu` is dead code (copying & pasting ?)

templates/repo/diff/image_diff.tmpl

@@ -2,7 +2,7 @@
 <tr>
 	<td colspan="2">
 		<div class="image-diff" data-path-before="{{.root.BeforeRawPath}}/{{PathEscapeSegments .file.OldName}}" data-path-after="{{.root.RawPath}}/{{PathEscapeSegments .file.Name}}">
-			<div class="ui secondary pointing tabular top attached borderless menu stackable new-menu">
+			<div class="ui secondary pointing tabular top attached borderless menu new-menu">
 				<div class="new-menu-inner">
 					<a class="item active" data-tab="diff-side-by-side-{{.file.Index}}">{{.root.locale.Tr "repo.diff.image.side_by_side"}}</a>
 					{{if and .blobBase .blobHead}}