Bug 35530: Tell UserCSS and UserJS in libraries administration are for OPAC

Bug 35530: Tell UserCSS and UserJS in libraries administration are for OPAC

In libraries administration new fields UserCSS and UserJS are for OPAC,
this should be in label.

This patch uses text "Specific OPAC CSS", since the system preference OPACUserCSS as help :
Library specific CSS can be defined in ...

Test plan
1) Go to Administration >  Libraries
2) Modify a library
3) Check you see new labels

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>