Handle hook signature change in Wagtail 2.10+

[chosak]

As of version 2.10, certain Wagtail hooks now accept an optional next_url keyword argument. This includes the register_page_listing_more_buttons hook used in this project.

Users of Wagtail 2.10 and 2.11 currently see this deprecation warning:

RemovedInWagtail212Warning: register_page_listing_more_buttons hooks will require an additional kwarg next_url in a future release. Please update your hook function to accept next_url.

This commit fixes that warning while still maintaining compatibility with the older versions of Wagtail.

Checklist

• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the front end playbook
• Passes all existing automated tests
• New functions include new tests
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged
• Visually tested in supported browsers and devices
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)