Refactor create test form

[jhonatan-lopes]

Fixes #79.

Modifies CreateAbTestForm and the respective add_form view so that the template is rendered via Wagtail's panels. This ensures visual consistency with current Wagtail UX/UI and also simplifies the template.

The React component responsible for choosing a page and a goal has also been updated with the 5.X styling.

[jhonatan-lopes]

We need to re-run the CI once #77 and #78 are merged as this will only work on Wagtail >= 5

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.44%. Comparing base (91d0e0c) to head (7e5bb95).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   88.40%   88.44%   +0.04%     
==========================================
  Files          15       15              
  Lines         750      753       +3     
  Branches      140      140              
==========================================
+ Hits          663      666       +3     
  Misses         51       51              
  Partials       36       36              

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[Stormheg]

This should fall back to an empty object instead of an empty string, otherwise JSON.parse will crash with SyntaxError: Unexpected end of JSON input.

Suggested change

	const goalSelectorProps = JSON.parse(document.getElementById('data-goal-selector-props')?.textContent || '');
	const goalSelectorProps = JSON.parse(document.getElementById('data-goal-selector-props')?.textContent || '{}');

[jhonatan-lopes]

Thanks!