Install GOV.UK Frontend v6 on the management pages

[owenatgov]

What

Install frontend v6 on the management pages. This includes removing isntances of functional colour variables and updating templates to use the v6 landmark API.

Why

So that the management pages get the benefits of frontend v6

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• v6 is installed on the kit
• functional colour sass variables are replaced with calls to govuk-functional-colour
• the new landmark API is applied to templates