Bug 28140: Accessibility: OPAC - "sort_by" select isn't labelled on search results...

Bug 28140: Accessibility: OPAC - "sort_by" select isn't labelled on search results page

This patch adds a label to the sort function on the OPAC
results page to provide context to screen readers and improve
accessibility.

The label is visually hidden and does not change how the page
displays.

To test:
- Search for something on the OPAC
- Change how the results are sorted and confirm sort still works
- Confirm that there are no visual changes to the page
- Inspect the sort function to confirm it now has a label
- Ideally confirm the label works with a screenreader

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>