Bug 23915: Replace OPAC list sort menu with Bootstrap menu button

Bug 23915: Replace OPAC list sort menu with Bootstrap menu button

This patch adds JavaScript to the list contents page which converts the
resort form's <select> menu into a Bootstrap dropdown menu. This allows
for a more compact and consistent display.

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

View a list in the OPAC. The toolbar of list controls ("New list,"
"Download list," etc.) should have a similary-styled "Sort" button. Each
sorting option should work correctly.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>