Bug 27631: Hidden <h1> on intranet-main.tt

Bug 27631: Hidden <h1> on intranet-main.tt

This patch visually hides a <h1> on intranet-main.tt so that the
heading is available to be read out by screenreaders in the summary of
headings on the page.

Test plan:

1) Ensure the <h1> "Koha home" heading on the staff client main/home
   page is hidden visually, but in the source code you can see it

Sponsored-by: Catalyst IT
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>