Link and Focus colour accessibility

[garethrees]

Extracted from #22 (comment)

• The green (#0aa44c) used for links fails colour contrast check (3.27:1) on a white background. If you darken it slightly (#08863e) it will pass

• The focus colour (#ffbf47) used for links fails colour contrast when used with the link green (#099846) - suggest we change the focus colour to black and keep the background colour as is

• The link colour (#a9cd75) used in the footer fails contrast check on the footer background colour. Lightening it a few steps (#c4dca3) fixes this

• The focus colour used for links in the footer, combined with the background fails contrast check

These colours come from https://github.com/unboxed/hackney_template, so we'll need to make an upstream issue.