Move header and footer blocks out of govuk-branded into main

[owenatgov]

Resolves #2481

[joelanman]

just wondering if this is the only/best way forward, as it will mean:

• everyone who upgrades will have to manually update their layouts/main.html file
• we can't even give universal instructions as we don't know what their layouts/main.html contains

[owenatgov]

@joelanman So, this is a tricky one.

The kit is impacted a lot by changes to our landmark API that are happening in v6. I did a detailed writeup in #2477 (comment) which includes the problem this is solving. There's a few angles of sumamry from that comment and generally where our heads are.

Firstly, the release context:

• A breaking release of the kit is deffo required for it to work with v6
• We want to avoid doing 2 breaking releases of both the kit and v6 around the same time period to minimise impact to kit users and diffuse the work required by those users to upgrade both to a new kit and a new version of frontend v6
• We can either make a breaking release that forces users to only use v6 of frontend to stay up to date with both or try to find a way for the kit to work across both versions, giving us more freedom in our release planning
• This change is trying to do the latter

Secondly our assessment of user interaction with the kit and frontend:

• User's are going to need to update their prototypes for v6 no matter what
• these blocks live in the kit package where users can't update them
• This is an attempt to at least allow new prototypes to have to do minimal work whilst avoiding hidden work for users upgrading to frontend v6

This plan could change. I thought about this briefly this morning and I'm debating if it might in fact be simpler to have 2 breaking releases at the same time after all. We're gonna be revisiting this in the new year with fresh heads to see what the most suitable option is.