Add hidden text to cookie banner buttons
Add hidden text to cookie banner buttons

Visual users get the context from the proximity to
the h2 but we need to provide this for others.

I've tried using `aria-describedby` to link them
to the h2 but this ends up making the h2 text the
button's description in the accessibility tree. In
Voiceover this means you only get that information
as extra context, announced a while after the
label if the VO cursor stays on the button.

We want all the information in the accessible
label so chose this approach instead.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin