Bug 28993: Switch magnifying glass in staff detail pages to FA icon

Bug 28993: Switch magnifying glass in staff detail pages to FA icon

This patch modifies the staff interface's bibliographic detail XSL so
that a Font Awesome is used instead of a magnifying glass image when
listing authority subject headings.

The patch also corrects two other places where the image is used so that
the image can be removed. Some markup from authorities.detail.tt is
removed because it appears to be unused (a copy and paste from
catalogue/detail.tt).

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

- To view the changes to the staff interface bibliographic detail page,
  locate a record which has at least one subject heading which is linked
  to a subject authority record.
- The subject heading link should be followed by a magifying glass
  (Font Awesome) icon which links to the authority record's detail page.
- View a similar change to the MARC detail page for the record. Confirm
  that the subject headings are followed by a link to the authority
  detail page.
- Follow the authority search link to locate and view an authority
  record. The information on the authority detail page should be
  correct.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>