Bug 22807: (follow-up) Update style of skip button

Bug 22807: (follow-up) Update style of skip button

This patch updates the markup and style of the skip button, adapting
(i.e. stealing) the style of the skip link at https://webaim.org.

To test, apply the patch and rebuild the OPAC CSS:
https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client

Go to any page in the OPAC and hit the tab key. The "Skip to main
content" link should appear. Tabbing away from it should cause the link
to hide. Hitting the ENTER key when the link is highlighted should cause
the page to scroll to the main content.

Signed-off-by: Hayley Mapley <hayleymapley@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>