Change domain flow improvements

[RobertJoonas]

Changes

Improve the domain change flow. Companion PR: plausible/docs#644

Some screen recordings and screenshots can be found in https://3.basecamp.com/5308029/buckets/42034199/card_tables/cards/9149604457

• Styling improvements:
  • Do not change the footer style between the form and success views
  • Remove the yellow background notice and replace with just a paragraph
  • Always replace the loading spinner with an icon and a message:
    • Detection failure -> ⚠️ We could not reach your new domain
    • Detection success: v1 | NPM | WP -> ⚠️ Additional action required
    • Detection success: no v1 detected -> ✅ Your new domain should be tracking nicely
• Always render a link in the footer to the domain change documentation
• When detection fails, render a more generic notice (additional action required -> additonal action may be required)
• For a successful detection where no further action is required, link to a new section introduced in the docs PR - the domain change checklist

Tests

• Automated tests have been adjusted

Changelog

• This PR does not make a user-facing change

Documentation

• Domain change improvements docs#644

Dark mode

• The UI has been tested both in dark and light mode

[apata]

Thanks for taking the time to refine this! I shared some nitpicks, but overall I think you've clarified things quite a bit.