Bug 25155: Fixing semantic headings login modal

Bug 25155: Fixing semantic headings login modal

The login modal headings were semantically incorrect prior to this
patch.

Test plan:
1) Click the 'Log in to your account' option to expose the modal
2) Use a headings inspector and note we jump from H1 (Page Title) -> H3
   (Modal Title)
3) Apply the patch and reload the page.
4) Click the 'Log in to your account' option to expose the modal
5) The modal should still appear well formed
6) Use a headings inspector and note we jump from H1 (Page Title) -> H2
   (Modal Title) -> H3 (Shibboleth title if enabled)

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>