Bug 27889: Adjust responsive width of OPAC advanced search form

Bug 27889: Adjust responsive width of OPAC advanced search form

This patch tweaks the CSS for the advanced search form in the OPAC so
that it adjusts well at various browser widths, including preventing the
form from taking up the whole width of the page at higher browser
widths.

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

- In the OPAC, go to the advanced search page.
- Test the appearance of the fields in the first "Search for" section,
  confirming that everything adjusts well at all widths large and small.
- Click the "More options" button and test that configuration as well.

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>