Bug 27889: (follow-up) Adjust width of OpacAddMastheadLibraryPulldown

Bug 27889: (follow-up) Adjust width of OpacAddMastheadLibraryPulldown

This patch adds some responsive classes to the
OpacAddMastheadLibraryPulldown container so that it will adjust better
at various browser widths.

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>
(cherry picked from commit 147b1ced1dee259c4f210d88a5ec902b549d2836)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>