Bug 19851: Improve responsive layout handling of staff client menu bar

Bug 19851: Improve responsive layout handling of staff client menu bar

This patch modifies CSS and markup in the staff client header to
override the default responsive style of the menu bar. The changed
layout will keep the menu bar items in a horizontal line at lower screen
widths.

To test, apply the patch and clear your browser cache if necessary.

 - Log in to the staff client and confirm that the appearance of the
   menu bar at the top of the screen looks correct at various browser
   widths.
 - Confirm that the logged-in user menu label and help link expand and
   collapse according to browser width.

Signed-off-by: Dominic Pichette <dominic.pichette@inlibro.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>