Bug 26694: Set focus for cursor to search input box on guarantor_search.pl

Bug 26694: Set focus for cursor to search input box on guarantor_search.pl

This patch adds the "focus" class to the patron search field so that
cursor focus is set when the page loads.

To test, apply the patch and view an adult patron record in the staff
client.

- Click the "Add guaratee" button in the patron toolbar.
- On the following "Add patron" page, click the "Search to add" button
  in the "Patron guarantor" section.
- A popup window should appear, and the focus should automatically be on
  the first form field.

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>