Wagtail 6: Switch News-and-Updates and Authors sidebar-menus from Snippets to more fitting Viewset options

[johnnyporkchops]

Blocking issue:
#6416

Summary

• In this commit, we were forced to temporarily switch the News and Updates and Authors sidebar-menu items from the ModelAdmin to Snippets because of issues with the ModelAdmin. (Since wagtail.contrib.modeladmin is an installed package in maintenance-mode that will not receive further updates, we removed it from settings/base.py)
  • Neither of these are Snippets and it would be confusing to editors to keep them in the Sniippet admin area
• Prior to to Wagrail 6.1, Snippets was the suggested temporary option. Once we have upgraded, more fitting Viewset options are available for both.

Technical steps:*

• Switch News and Updates sidebar-menu from Snippets to PageListingViewsets in a ViewSetGroup
• Switch Authors sidebar-menu to ModelViewset
• We can remove index_custom_button.html, this was a fix to avoid the "choose a parent" button that caused the page timeouts in production. In Wagtail 6, "choose a parent" does not timeout because it presents a page explorer instead of trying to list all 10,000+ pages in the tree.

*John has a saved version of wagtail_hooks.py with most of the necessary changes

Completion Criteria:

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