userportal,webadmin: Grid drag 'jump'

- When starting to drag a column in a grid, the line separating
  the column you are dragging would jump a few pixels. This patch
  solves that issue.

Change-Id: If87c7c8fbd037cf5b48c45814690868123deb188
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1139741
Signed-off-by: Alexander Wels <awels@redhat.com>