Fix tasks #6637: fix z-index issue on IE9/Chrome for dropdown menu

Change-Id: I69d62bb56696971833d3dafb233c09b9c153ec24
Merge commit 'refs/changes/70/2270/1' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix tasks #6637: fix z-index issue on IE9/Chrome for dropdown menu
This is Tuleap 6.12.99.28

Change-Id: I4a31e40527a04d00a3570b80a7769f71f916796c