Bug 22807: (follow-up) Add .maincontent class to libraries page

Bug 22807: (follow-up) Add .maincontent class to libraries page

This patch adds a missing "maincontent" class to the "all libraries"
view. Previously it was only present on the single library view.

To test, apply the patch and go to the "Libraries" page in the OPAC.

 - Hit the "tab" key to highlight the "Skip to main content" link.
 - Hit "Enter."
 - The page should scroll to the top of the list of libraries.
 - Open one of the library detail pages and confirm that the "Skip to
   main content" link works on that page too.

Signed-off-by: Hayley Mapley <hayleymapley@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>