Add cookie banner component

[alex-ju]

Need

We need to update the cookie banner which is currently part of govuk_template. There was an attempt to do this update in govuk_template, but since that's a deprecated repository we shouldn't do any new work or releases on it.

Instead, this PR moves the remaining govuk-template.js scripts into app/assets/javascripts/govuk_publishing_components/lib/ and extracts the cookie banner into its own component.

Complementary work

A follow up PR will come after this, updating static to remove govuk-template.js and the associated markup for the cookie banner and replace it with this component.

Update: PR in static alphagov/static#1662

Trello card

[andysellick]

Just a few minor comments, also does this need more tests? I thinking of the JS in the first commit and a spec test for the component to check that clicking the link makes the banner disappear?

[alex-ju]

Thanks for the review @andysellick! I addressed all the comments including tests for the cookie banner visibility and the cookie state.

[fofr]

Can I ask why a "hide this message" link has been added?

Edit: Looks like the cookie bar is now persistent until it’s dismissed, which explains the new link. For services who will want to be consistent, what's the reasoning behind this?

[alex-ju]

This update is part of a GOV.UK Cookie & Tracking Policy mission in an effort to align it with the spirit of the EU cookie and tracking policy legislation. To play back team's PMs reply to this question: "it’s a temporary change to increase visibility of the message while we consider other options in the next few months. that work will include looking at how this could be expanded across government."