DEV: Introduce <DPageHeader /> and <DPageSubheader /> components (#30146)

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

plugins/chat/admin/assets/javascripts/discourse/templates/admin-plugins/show/discourse-chat-incoming-webhooks/index.hbs

@@ -4,9 +4,9 @@
 />
 
 <div class="discourse-chat-incoming-webhooks admin-detail">
-  <AdminPageSubheader
-    @titleLabel="chat.incoming_webhooks.title"
-    @descriptionLabel="chat.incoming_webhooks.instructions"
+  <DPageSubheader
+    @titleLabel={{i18n "chat.incoming_webhooks.title"}}
+    @descriptionLabel={{i18n "chat.incoming_webhooks.instructions"}}
   >
     <:actions as |actions|>
       <actions.Primary
@@ -18,7 +18,7 @@
         class="admin-incoming-webhooks-new"
       />
     </:actions>
-  </AdminPageSubheader>
+  </DPageSubheader>
 
   <div class="incoming-chat-webhooks">
     {{#if this.model.incoming_chat_webhooks}}