Bug 20051: Invalid markup in staff client's header.inc

Bug 20051: Invalid markup in staff client's header.inc

This patch corrects markup errors in header.inc which introduced by
Bug 19851. For id attributes on <span>s have been replaced with classes,
eliminating an HTML validator error about duplicate ids.

Minor unrelated change: Updated an element's id with a more descriptive
name.

Test plan the same as Bug 19851's:

 - 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: Charles Farmer <charles.farmer@inLibro.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Fix for QA: Corrected ID reference in circ/offline.tt

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>