Switch slim sidebar to be on by default

[thibaudcolas]

Changes our settings so the slim sidebar is on by default. Fixes #5337. This includes:

• Changing how the feature flag works so it’s opt-out rather than opt-in, with a new WAGTAIL_SLIM_SIDEBAR setting to opt out.
• Updating the documentation to match what the slim sidebar provides.
• Updating the release notes, with upgrade consideration notes for anything that’s no longer supported.
• Updating our tests which relied on implementation details of the sidebar.

For this last point, I decided to refactor how the sidebar’s data is transferred client-side a bit, moving to a separate JSON script tag so it’s more readable when troubleshooting and when used in tests.

I’ve also updated the RemovedInWagtail217Warning I had added, as indeed upon checking with our deprecation policy it should be RemovedInWagtail218Warning.

---

• Do the tests still pass? (https://docs.wagtail.io/en/latest/contributing/developing.html#testing)
• Does the code comply with the style guide? (Run make lint from the Wagtail root)
• For Python changes: Have you added tests to cover the new/fixed behaviour?
• For front-end changes: Did you test on all of Wagtail’s supported environments?
  • Please list the exact browser and operating system versions you tested. Chrome, Safari macOS
  • Please list which assistive technologies you tested. N/A
• For new features: Has the documentation been updated accordingly?

[squash-labs]

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Test this branch here: https://thibaudcolasfeaturesidebar-def-xmhdz.squash.io

[kaedroho]

This looks good to me, thanks!