Bug 25834: Relabel "Search to add" to "Add guarantor" on patron form.

Bug 25834: Relabel "Search to add" to "Add guarantor" on patron form.

The "search to add" label on the button for adding guarantors is a bit
hard to translate and is inaccessable. This patch updated the button
label to "Add guarantor" to fix these issues.

To test:
1. Go to patrons
2. Add a new child patron
3. Observe the button with "search to add"
4. Save the patron
5. Edit the patron and observe the "search to add" button.
6. Apply patch.
7. Add a new patron
8. Observe new text on button.
9. Edit a child patron
10. Observe new text on button
11. Sign off

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>