Add cookie consent functions in the frontend docs

[vanitabarrett]

Follows on from alphagov/govuk-design-system#1614

What

In alphagov/govuk-design-system#1614, we added a set of cookie consent functions to the design system. We need to add the same set of functions to the frontend docs.

Why

We will need this set of functions to hook into when we add the cookie banner and cookie preferences form.

Who needs to know about this

Developers

Done when

• We have functions for the main cookie banner/form behaviour, e.g: accepting and rejecting cookies; deleting cookies (see list in design system card)
• Tests for all of the cookie consent functions
• Consent cookie can be shared with design system site
• Functions should match what's in the design system

[stevenjmesser]

Closing as we don't have any tracking on the Frontend docs currently. (We could implement cookie-less analytics if useful.)

[stevenjmesser]

Closed accidentally, it's part of #133.

[owenatgov]

See #134 (comment) on #134 as there's a lot of corssover between these issues.