Bug 26752: Convert OPAC list download modal to dropdown

Bug 26752: Convert OPAC list download modal to dropdown

This patch modifies the OPAC list page so that download options are
shown in a dropdown menu instead of a modal dialog. This solves a bug
introduced by the Bootstrap 4 upgrade and makes it consistent with the
cart interface.

To test, apply the patch and view the contents of a list in the OPAC
which has multiple items on it.

 - In the toolbar at the top of the list contents table the "Download"
   button should now be a dropdown menu.
 - Selecting any of the download options should trigger a download
   prompt.
 - Test with and without some CSV profiles which have been
   configured for use in the OPAC

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>