Bug 35260: Revise structure of the batch checkout page

Bug 35260: Revise structure of the batch checkout page

This patch makes some changes to the style and structure of the batch
checkout page in order to make it more usable.

To test, apply the patch and rebuild the staff interface CSS. Clear
your browser cache if necessary.

- Make sure the BatchCheckouts preference is enabled and that the
  BatchCheckoutsValidCategories has some categories selected.
- Locate a patron and open the batch checkout page.
- Test the process of checking out to the patron both by entering
  barcodes into the textarea and by uploading a file.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit b54da05c73c9ea5b04d3552b34fcc636b0a71851)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>