Bug 30457: (follow-up) Tweak markup around search form

Bug 30457: (follow-up) Tweak markup around search form

This patch makes a change to the markup around the patron/clubs search
form on the holds page. This change prevents the Bootstrap tabs from
inheriting an unwanted margin from the fieldset.brief definition.

The change also replaces a <label> tag with an <h2>. I think a heading
makes more sense than a free-floating <label>.

To test, apply the patch and confirm that the holds page
(/cgi-bin/koha/reserve/request.pl?biblionumber=XXX) looks correct.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>