Bug 25984: Add minimum height and width to shelf browser covers

Bug 25984: Add minimum height and width to shelf browser covers

This patch adds a class to the shelf browser cover image links and a
style for these links defining a minimum height and width. This allows
the browser's default tab focus outline to be visible.

This patch also adds a check for an enabled cover image service before
displaying the cover image links.

This patch introduces whitespace changes so diff accordingly.

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

- Enable the OPACShelfBrowser system preference.
- Enable at least one cover image service (Amazon, Google, Coce, etc).
- Locate a title in the OPAC which has items attached.
- On the bibliographic detail page, click the "Browse shelf" link in the
  table of holdings to display the shelf browser.
- Use the tab key to move the focus to cover images in the shelf
  browser.
- Confirm that the focus outline moves through each cover link whether
  or not there is a cover image available.
- Diable all cover image services.
- Test the shelf browser again and confirm that it's not necessary to
  tap through an empty row.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>