Bug 12291: Replace acronym with tag at the OPAC

Bug 12291: Replace acronym with tag at the OPAC

On the OPAC authentification page, the acronym tag is used, but
deprecated.
It should be replaced with abbr:
http://www.w3.org/wiki/HTML/Elements/acronym

Test plan:
Enable the CAS authentification and verify the tags have correctly been
replaced.

Signed-off-by: Magnus Enger <magnus@enger.priv.no>
Befor the patch, "grep -r "<acronym" koha-tmpl/*/*/en/*" shows 4 occurences of
<acronym>, 3 related to CAS. After the patch, the same grep shows 1 remaining
<acronym>, in acqui/parcels.tt. I sign off this patch and will provide a
followup for the last occurrence.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>