WIP: Add wagtail AB testing package

[helenb]

Link to Ticket

Description of Changes Made

Adds the wagtail AB testing package. Unfortunately it isn't functioning correctly at the moment due to JavaScript errors when creating the test - see wagtail-nest/wagtail-ab-testing#84

To do:

• Re-test once the issue has been fixed
• Add the cloudflare JS required for this to work on prod

How to Test

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MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Tests and linting passes.

Unit tests

• Added
• Not required

Documentation

• Updated build docs
• Updated editor guidelines (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only)
• Updated field spec (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only)
• Not required

Browser testing

• I have tested in the following browsers and environments (edit the list as required)
  • Latest version of Chrome on mac
  • Latest version of Firefox on mac
  • Latest version of Safari on mac
  • Safari on last two versions of iOS
  • Chrome on last two versions of Android
• Not required

Data protection

• Not relevant
• This adds new sources of PII and documents it and modifies Birdbath processors accordingly

Accessibility

• Automated WCAG 2.1 tests pass
• HTML validation passes
• Manual WCAG 2.1 tests completed
• I have tested in a screen reader
• I have tested in high-contrast mode
• Any animations removed for prefers-reduced-motion
• Not required

Sustainability

• Images are optimised and lazy-loading used where appropriate
• SVGs have been optimised
• Perfomance and transfer of data considered
• If JavaScript is needed alternatives have been considered
• Not required

Pattern library

• The pattern library component for this template displays correctly, and does not break parent templates
• The styleguide is updated if relevant
• Changes are not relevant the pattern library