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 are going to start making section landing pages
for admin for each sidebar section. This lays the framework
with routes and simple components that can be further
refined by a designer, but I have taken the base CSS from
AI which Kris made.
The initial section landing items will be used in AI to replace
the placeholders added in this commit b8b3c61451
