Bug 26421: Use Bootstrap screen reader text class for shelf browser messages

Bug 26421: Use Bootstrap screen reader text class for shelf browser messages

This patch udpates the markup added by Bug 26039, replacing the "hidden"
class, which is no longer functional with Bootstrap 4, with "sr-only," a
class designed specifically for screen reader text.

The patch also corrects a typo, "shelfbrowers" -> "shelf browser"

To test, apply the patch and make sure the OPACShelfBrowser system
preference is enabled.

View the detail page for a bibliographic record with items. In the
holdings table, you should see links labeled "Browse shelf."

When you open the shelf browser there should be a link at the top that
reads "Close shelf browser."

If you have access to a screen reader, test that the hidden text is
announced by the reader. "Browse shelf (Opens below)" and "Close shelf
browser (Hides shelf browser)."

Signed-off-by: Stina Hallin <stina.hallin@ub.lu.se>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>