Add shortcut for SRs to `Skip to content` for a11y

app/views/layouts/header/_default.html.haml

 %header.navbar.navbar-fixed-top.navbar-gitlab{ class: nav_header_class }
+  %a{ href: "#content-body", tabindex: "1", class: "sr-only gl-accessibility" } Skip to content
   %div{ class: "container-fluid" }
     .header-content
       %button.side-nav-toggle{ type: 'button', "aria-label" => "Toggle global navigation" }