Bug 12034: standardize alerts referring to lists and list items

The staff client lists template contains a warning which differs from
the same warning in the OPAC. The OPAC version is correct because it
refers to a list rather than a shelf. This patch copies the OPAC version
to the staff client template.

This patch also corrects a few of other instances where the incorrect
term "shelf" is used.

To test, apply the patch and view a list in the staff client which is
populated with items. Check the box next to one or more items on the
list and click the "Remove selected" button. You should see the message,
"Are you sure you want to remove these items from the list?"

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>

Work as described, no koha-qa errors

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>