Bug 25154: Add heading where content is introduced in opac-results.tt

Bug 25154: Add heading where content is introduced in opac-results.tt

Prior to this patch, the search results page did not contain a
semantically accurate heading to introduce he main content of the page.

Test plan
1/ Perform a search in the OPAC that yields results
2/ Inspect the page and note that the 'title' stating how many results
   are found is not semantically marked up as a heading to introduce the
   content.
3/ Apply the patch
4/ Reload the page
5/ Inspect the page and note that the 'title' stating how many results
   are found is now properly marked up as a second level heading and
   appropriately designated as the start of the 'maincontent' block with an
   associated '.maincontent' class.
6/ Perform a search which yields zero results
7/ Note that the 'No results found' title is properly marked up as a
   second level heading.
8/ Signoff

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>