This converts the `<AdminPageHeader />` component and the
`<AdminPageSubheader />` components into new components
that can be used outside of admin, and updates the CSS classes.
Also introduces a `<DPageActionButton />` component and child
components for the header action buttons.
I have to keep the old admin-only components around for
now until plugins are updated, then we can remove it,
and remove the re-exports that are done within
admin-page-action-button.gjs
We want to allow admins to make new required fields apply to existing users. In order for this to work we need to have a way to make those users fill up the fields on their next page load. This is very similar to how adding a 2FA requirement post-fact works. Users will be redirected to a page where they can fill up the remaining required fields, and until they do that they won't be able to do anything else.
We're planning to implement a feature that allows adding required fields for existing users. This PR does some preparatory refactoring to make that possible. There should be no changes to existing behaviour. Just a small update to the admin UI.
