Bug 18394: Add an option to item search to export a barcodes file

Bug 18394: Add an option to item search to export a barcodes file

Test plan:
1. Go to item search (/cgi-bin/koha/catalogue/itemsearch.pl)
2. Fill the form with whatever you want
3. Leave the 'Output' option to 'Screen' and click 'Search'
4. Verify that the search still works
5. Click on 'Edit search' and set 'Output' to 'Barcodes file', click 'Search'
6. You should be able to download a 'barcodes.txt' file, open it and compare it
   to the previous search results
7. Now click on the 'Export results to barcodes file' button above the results,
   you should have the same result as in step 5
8. Verify that the CSV export still works

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>