Remove the `header` and `footer` blocks from the `govuk-branded` template

[owenatgov]

What

Remove the header and footer tempalte blocks from the govuk-branded template and move their declarations to the prototype starter layout.

Important

This is a breaking change

Why

Follows investigation from #2477

We will be significantly changing the GOV.UK Frontend landmark API in frontend v6. Moving these blocks means that users can use the branded template with both v5 and v6 when it's released.

Who needs to work on this

Developers

Who needs to review this

Developers