Bug 10346 - "Add multiple copies" should be labelled "Add multiple items"

The Koha interface standardizes on the term "item" instead of "copy," so
the "Add multiple copies" button would be more correctly labeled "Add
multiple items."

To test, view the add item screen for an existing or new record. You
should see a button labeled "Add multiple items."

Signed-off-by: Cedric Vita <cedric.vita@dracenie.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>