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>
(cherry picked from commit 513b226873759ba3df6765dce635fe96cf5a51f3)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 482fc13558437013479b31e7a553fd0919ee97c5)

Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
(cherry picked from commit 68f16bb3401ee825425fc115d8ace5b525af37c9)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>