Bug 6536: Add a form for SRU Field mapping

Bug 6536: Add a form for SRU Field mapping

This form allows you to enter the match between the standard names of
the Z3950 search fields and the field names for a specific SRU server.
This patch only adds the form; it will be used in a subsequent patch.

Test plan:
Not needed, but you can call the form by constructing a URL like:
    cgi-bin/koha/admin/sru_modmapping.pl?mapping=title=1,author=2
You should see the form with title and author filled.
Note that you cannot save the information yet.

Signed-off-by: Giuseppe Angilella <giuseppe.angilella@ct.infn.it>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>