Bug 27610: (follow-up) Hide <h1> on opac-main.tt

Bug 27610: (follow-up) Hide <h1> on opac-main.tt

This patch visually hides the <h1> on opac-main.tt so that libraries' customisations of their home pages are not disturbed by the introduction of a descriptive <h1>, but the heading is still available to be read out by screenreaders in the summary of headings on the page.

Test plan:

1) Apply dependencies and all patches on this bug report
3) Ensure the <h1> "Koha home" heading on the OPAC main/home page is hidden visually, but in the source code you can see it

Sponsored-by: Catalyst IT
Signed-off-by: Henry Bolshaw <bolshawh@parliament.uk>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>