Upgrade Wagtail to 4.2 (#3045)

Fixes #3018

- Update requirements - `wagtail` and `wagtail-cache`
- Update `BaseSetting` to `BaseSiteSettings`
- Fix Category StreamBlock at hypha/apply/categories/blocks.py
- Add migration changes
- Add `wagtail_reference_index_ignore = True` to models (See [this
issue](https://github.com/wagtail/wagtail/issues/9731

) on wagtail for
more info)
- Use checkboxes when selecting reviewers for a Round and a Lab
- Update `ReadOnlyPanel` to use `render_html()` method instead of
`render_as_object()`

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Co-authored-by: Fredrik Jonsson <frjo@xdeb.org>