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 ?)

web_src/css/base.css

@@ -1915,7 +1915,7 @@ img.ui.avatar,
 }
 
 .ui.menu.new-menu .item {
-  margin: 0 !important;
+  margin-bottom: 0 !important; /* reset fomantic's margin, because the active menu has special bottom border */
 }
 
 @media (max-width: 767px) {
@@ -1924,8 +1924,13 @@ img.ui.avatar,
   }
 }
 
+.ui.menu.new-menu .item:first-child {
+  margin-left: auto; /* "justify-content: center" doesn't work with "overflow: auto", so use margin: auto */
+}
+
 .ui.menu.new-menu .item:last-child {
   padding-right: 30px !important;
+  margin-right: auto;
 }
 
 .ui.menu.new-menu::-webkit-scrollbar {

web_src/css/explore.css

@@ -1,5 +1,4 @@
 .explore .navbar {
-  justify-content: center;
   margin-bottom: 15px !important;
   background-color: var(--color-navbar) !important;
   border-width: 1px !important;

web_src/css/runner.css

@@ -32,10 +32,6 @@
   color: var(--color-white) !important;
 }
 
-.runner-container .runner-new-menu {
-  width: 300px;
-}
-
 .runner-container .task-status-success {
   background-color: var(--color-green);
   color: var(--color-white);