Bug 35206: Adjust style of add button on curbside pickups administration

Bug 35206: Adjust style of add button on curbside pickups administration

This patch adds Bootstrap classes to the "Add" button on the curbside
administration page, in the "Curbside pickup hours" section. This makes
it consistent with the markup of similar buttons in the interface and
makes it slightly smaller so that it fits better in the form.

To test, apply the patch and make sure the "CurbsidePickup" preference
is enabled.

- Go to Administration -> Curbside pickups.
- Fill out the form, including the "New slot" fields in the "Curbside
  pickup hours" section.
  - Confirm that the corresponding "Add" button looks correct, and that
    it works to add your new slot.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>