Bug 21330: Allow XSLT for authority detail view in OPAC

Bug 21330: Allow XSLT for authority detail view in OPAC

This patch adds a syspref that allow to customize the authority detail
view in OPAC with XSLT.

Test plan:
1. Make sure to have at least one or more authorities
2. OPAC: Home > Authority search(Submit) > Authority search results
3. Click details on a result and notice the view
4. Apply patch
5. INTRA: Home > Administration > System preferences ->find
   "AuthorityXSLTOpacDetailsDisplay"
6. Write the path where your file is. You can try with the XSLT for
   biblio for instance:
   .../koha/koha-tmpl/opac-tmpl/bootstrap/en/xslt/UNIMARCslim2OPACDetail.xsl
7. Save changes
8. Repeat 2-3 and notice the display

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Thibault <thibault.keromnes@univ-paris8.fr>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>