Don't prevent overflow on y-as (go-gitea#19978)

- Fomantic tries to prevent overflowing on the `y/x`-as by default on
stackable menu's on mobile screens. We already solve this issue by
forcing overflow on x as and hide it on y as(due to some issues with
other menu's), since go-gitea#19486.
- However this edge case does require a y-overflow to show the dropdown,
because you cannot easily adjust this with CSS, once you're fiddling
with overflow's (https://stackoverflow.com/a/6433475). However
interesting behavior is noted
https://css-tricks.com/popping-hidden-overflow/ when you remove the
position: relative, it will suddenly work again. Well because this is
the only solution without redesigning dropdowns, I think we can live
with the side-effect of the dropdown items being full-width instead
"relative" width to their parent.
- Resolves go-gitea#19976

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/_base.less

@@ -2200,5 +2200,9 @@ table th[data-sortt-desc] {
     .item {
       width: initial !important;
     }
+
+    > .dropdown.item {
+      position: initial;
+    }
   }
 }