Bug 25892: Clarify the visual hierarchy of holds by branch and itemtype

Bug 25892: Clarify the visual hierarchy of holds by branch and itemtype

This patch makes minor changes to the markup of the holds list to make
the page hierarchy clearer when holds are grouped by branch and
itemtype.

Test plan:

 - Enable the  AllowHoldItemTypeSelection system preference.
 - Locate a record in the catalog and place multiple holds:
   - Holds at different branches
   - Holds with different itemtypes selected
 - View the holds page with various values of the HoldsSplitQueue
   preference enabled:
   - nothing
   - pickup library
   - hold itemtype
   - pickup library & itemtype

   In each case verify that the interface works well.

Signed-off-by: Christopher Brannon <cbrannon@cdalibrary.org>
Signed-off-by: Emmi Takkinen <emmi.takkinen@outlook.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>