Bug 30740: Link to authorities 'used in' should not use equal

Bug 30740: Link to authorities 'used in' should not use equal

When showing an authority, there is a link to search biblio records it is used in.
Actually this link uses 'q=an=$auth_id', this is wrong for an URL, and other places use 'idx:value'.
It does not break search with Zebra nor Elasticsearch but it is better to use ':'

Test plan :
Check changed links return same results with and without patch

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