Fix swimlanes sorting

Since 7cc185ac "Properly fix horizontal rendering on Chrome and Firefox"

The rendering of the new design of the swimlanes was correct, but this
commit broke the reordering capability. Having the swimlane header at
the same level than the lists of cards makes the whole sortable
pattern fail.

2 solutions:
- revert to only have 1 div per swimlane. But this introduces the firefox
  bug mentioned in 7cc185ac, so not ideal
- force the sortable pattern to do what we want.

To force the sortable pattern, we need:
- add in the helper a clone of the list of cards (to not just move the
  header)
- make sure the placeholder never get placed between the header and the
  list of cards in a swimlane
- fix the finding of the next and previous list of cards.

For all of this to be successful, we need to resize the swimlanes to a
known value. This can lead to some visual jumps with scrolling when you
drag or drop the swimlanea. I tried to remedy that by computing the new
scroll value. Still not ideal however, as there are still some jumps when
dropping.

Fixes #2159

client/components/swimlanes/swimlanes.styl

@@ -53,6 +53,9 @@
 .list-group
   height: 100%
 
+.moving-swimlane
+  display: none
+
 swimlane-color(background, color...)
   background: background !important
   if color