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/explore/navbar.tmpl

@@ -1,4 +1,4 @@
-<div class="ui secondary pointing tabular top attached borderless stackable menu new-menu navbar">
+<div class="ui secondary pointing tabular top attached borderless menu new-menu navbar">
 	<a class="{{if .PageIsExploreRepositories}}active {{end}}item" href="{{AppSubUrl}}/explore/repos">
 		{{svg "octicon-repo"}} {{.locale.Tr "explore.repos"}}
 	</a>

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}}

templates/user/auth/signup_openid_navbar.tmpl

@@ -1,4 +1,4 @@
-<div class="ui secondary pointing tabular top attached borderless menu stackable new-menu navbar">
+<div class="ui secondary pointing tabular top attached borderless menu new-menu navbar">
 	<div class="new-menu-inner">
 		<a class="{{if .PageIsOpenIDConnect}}active {{end}}item" href="{{AppSubUrl}}/user/openid/connect">
 			{{.locale.Tr "auth.openid_connect_title"}}