Bug 25155: Fixing semantic headings in opac-auth.tt

Bug 25155: Fixing semantic headings in opac-auth.tt

Test plan:
1) Follow test plan from comment #1
2) Apply the patch
3) Reload the page, and note that is still displays fine
4) Check that the main heading is now an h1, and that the other headings
on the page make sense

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>