ui: fix rendering issue on firefox

When a list have more cards that can fit in the screen, the members icons
are drawn on top of the swimlane below and the scrollbar is not available.

According to https://github.com/utatti/perfect-scrollbar the container
must have an `overflow: hidden` css style.

When changing the swimlane header from vertical to horizontal, dd88eb4cc19
broke this which led to this weird bug.

client/components/swimlanes/swimlanes.styl

@@ -53,6 +53,7 @@
 .list-group
   flex-direction: row
   height: 100%
+  overflow: hidden
 
 swimlane-color(background, color...)
   background: background !important