Never warp navigation items

Otherwise it might happen that some menu item just disappears to
overwrite:hidden area. It's better to show at least part of it (and
ideally customize the widths so that it fits into the screen).

This commit fixes a problem, when 'Changeset Management' navigation is
not visible when on Content tab.
Merge pull request #1742 from iNecas/nowrap-nav

Never warp navigation items