Bug 18860: Adding thead tags to OPAC messaging settings table

Bug 18860: Adding thead tags to OPAC messaging settings 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:
thead {
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 your messaging. Notice this table's headings are not styled
with red text
6) Apply patch and refresh page
7) Your messaging table should now have red headings.
(You can delete the CSS from OPACUserCSS if you'd like.)

Sponsored-by: Catalyst IT
Signed-off-by: Jesse Maseto <jesse@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>