Bug 30554: Use XSLT (if enabled) in authority search cataloguing plugin

Bug 30554: Use XSLT (if enabled) in authority search cataloguing plugin

Test plan:
1. Set AuthorityXSLTResultsDisplay to a valid XSL file (see bug 30554
   attachments, there is an example XSL file)
2. Go to the bibliographic record editor (edit an existing one or create
   a new one)
3. Find a field linked to authorities and open the plugin
4. Start a search and verify that the results are displayed using the
   XSLT output (if using the example file it should be a link saying
   "Authority #<authid>")

Sponsored-by: Écoles nationales supérieure d'architecture (ENSA)
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>