#3198905 Gutenboarding: update domain picker styling

Changes proposed in this Pull Request

Quoting #40742 (comment):

Update from today. I’ve done the following:

• Restyled the suggestion items as radio input fields, with checked, focus, and hover states
• Updated the domain form to use onDomainSelect when selecting a paid domain instead of onDomainPurchase (this will require the rest of the domain flow to be in place as it means cutting off the current domain flow), but it ensures that selecting a domain works as expected
• Add a popover container so that the popover can be fixed in position matching the positioning in the Figma — this ensures that the popover doesn’t move around as the user selects their domain and the text is updated in the header
• Added search icon
• Added close button

There’s still a bit to be tidied up — the spacing is approximate in places, so might need to be double-checked / tweaked against the Figma. And the spacing in the loading state needs to be adjusted, as it’s current too short. I also haven’t addressed any mobile styling.

(Note by @ockham — I think we can tweak the above in a follow-up.)

Testing instructions

• http://calypso.localhost:3000/gutenboarding
• Verify that the domain picker looks as in the above screenshot, and that it still works.