Bug 38251: Make 'Remove selected items' button work when there is one item

Bug 38251: Make 'Remove selected items' button work when there is one item

There are two ways to remove items from a list: you can remove a single item
by clicking a 'Remove from this list' button below the item, or you can
remove one or more items by checking the checkboxes for them and clicking the
'Remove selected items' button.

Bug 37150 needed to fix 'Remove from this list' for CSRF, because in the wild
old days it just triggered a GET to delete, and instead it needed to POST.
The first patch to do that defined the JS var single_bib, checked the
checkbox for that bib and submitted the form, as though you had used the
checkbox yourself. But if you cancelled the confirmation prompt, it would
need to uncheck the checkbox it checked, so it added a function to do that
if (single_bib). That patch didn't land.

What landed instead was a patch with a separate hidden form that could be
populated with the single biblionumber from 'Remove from this list', but
without single_bib to define that state, it used selected_titles.size() == 1
as the condition to use the hidden form, forgetting that when the user checks
a single checkbox, selected_titles.size() is also 1. The right condition is
the same one used to tell where to get the titles for the confirmation dialog:
if the click event passed along a biblionumber, it's 'Remove from this list.'
Since checking that sets var biblionumber, we can just use that being truthy
as the condition.

Test plan:
 1. Without the patch, open the OPAC and log in
 2. Add any two biblios to a new list
 3. Lists - {your list name}
 4. Click the checkbox for one of the items, click 'Remove selected items'
    and click 'Yes, remove from list' in the confirmation dialog
 5. Note that it wasn't removed
 6. Click the checkboxes for both items, click 'Remove selected items' and
    click 'Yes, remove from list' in the confirmation dialog
 7. Note that both were removed
 8. Apply patch, restart_all
 9. Add any four items to your list
10. Lists - {your list name}
11. For the first item, click the 'Remove from this list' button below the
    details for the item, confirm, make sure it was deleted
12. Click the checkbox for what's now the first item, click 'Remove from
    this list,' confirm, make sure it was deleted
13. Click the checkboxes for what are now the first two items, click 'Remove
    from this list,' confirm, make sure both were deleted

Sponsored-by: Chetco Community Public Library
Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>