Currently it’s not possible for a screen reader user to know which
financial year they’re looking at. From the accessibility report:
> The financial year links are contained in a navigation region -
> tabbing or arrowing through only reads out the links, not the main
> information of "2016 to 2017 financial year" - that information is
> vital for understanding the page content.
This problem also applies to other pages which use the `pill` component,
which is effectively tabbed navigation (that reloads the page rather
than showing or hiding content on the page).
There are specific ARIA attributes that can be used to mark up a
navigation as being tabbed. This commit:
- adds those attributes
- makes the selected ‘tab’ visible to screenreaders and keyboard
focusable
- adds a visual focus indicator to the selected tab
- adds `id`s to the parts of the page that are controlled by the tabs so
that they are labelled as such
This also means changing the pill component from being a `<nav>` to a
`<ul>` because `tablist` is not a valid `role` for a `nav`.
Mostly follows the example here:
http://accessibility.athena-ict.com/aria/examples/tabpanel2.shtml
