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.
Benjamin Tissoires