Bug 22807: Add 'Skip to content' option

Bug 22807: Add 'Skip to content' option

This patch adds a 'Skip to content' link to the header bar which will
only appear upon the first use of 'tab' to navigate after any fresh page
load in the OPAC.

Test plan
1/ Load any page in the OPAC
2/ Hit the `tab` key
3/ Note the new 'Skip to main content' link appears at the top left of
the screen.
4/ Hit `Enter` or Click the button
5/ Note the page scrolls to the area of the page that has the first block
   containing a .maincontent class.
6/ Note that the next available focusable element after the first .maincontent
   block has been given focus.
7/ Note that the link has been hidden

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>