Bug 18949: Adding thead tags to OPAC MARC details holdings table

Bug 18949: Adding thead tags to OPAC MARC details holdings table

So that it can be styled alongside other tables in the OPAC.

To test:
1) Log into staff side
2) Find OPACUserCSS syspref and add the following CSS:
th,
.table-bordered>thead>tr:first-child>th {
color: red;
}
3) Log into the OPAC
4) Go to your fines, your search history, your reading history etc -
notice all of these table's headings are styled with red text
5) Go to the detail page of an item, then the MARC view.
Notice this table's headings are now styled with red text
6) Apply patch and refresh page
7) Your holdings table should now have red headings.
(You can delete the CSS from OPACUserCSS if you'd like.)

Sponsored-by: Catalyst IT
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>