Bug 31936: Restore link to advanced search in acquisitions header

Bug 31936: Restore link to advanced search in acquisitions header

This patch adds back the link to the advanced order search page which
was previously in the acquisitions header search. Some CSS is tweaked to
make the form style comfortable.

To test, apply the patch and rebuild the staff interface CSS.

- Go to Acquisitions.
- In the header search form, click "Orders search"
- Click the icon in the search form to expand the additional options.
- You should see an "Advanced search" link which is the same style as
  other standard links.
- The link should take you to the order search form.

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