Bug 34616: Add native bootstrap behaviour to default server dialog

Bug 34616: Add native bootstrap behaviour to default server dialog

Test plan:
Create a new SMTP server
Tick the 'default server' checkbox
Verify it now has a backdrop, closes if u click the backdrop or press 'esc'
The Yes and No buttons are now styled properly

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit b9d3997063344dcd04f6fac4810a9d3576c9e4c2)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>