UX: Automatically collapse admin page header buttons on mobile (#29040)

This commit attempts to improve the mobile experience for
admin page header and subheader by automatically collapsing
all action buttons in these components into a DMenu when viewing
mobile.

This is done by using different "list" wrapper components and a
DMenu trigger and a DropdownMenu on mobile only, and uses has-block
to determine whether to render the DMenu trigger at all.

This also removes the `PluginOutlet` in `AdminPluginConfigPage`, it
was too inflexible for this `DropdownMenu` case, and since the `:actions`
were always rendering we couldn't rely on `has-block`. A new plugin API,
`registerPluginHeaderActionComponent`, has been introduced instead to
replace it.

plugins/chat/admin/assets/javascripts/admin/components/chat-admin-plugin-actions.gjs

@@ -29,10 +29,11 @@ export default class ChatAdminPluginActions extends Component {
   }
 
   <template>
-    <@outletArgs.actions.Primary
+    <@actions.Primary
       @label="chat.admin.export_messages.create_export"
       @title="chat.admin.export_messages.create_export"
       @action={{this.confirmExportMessages}}
+      @icon="right-from-bracket"
       class="admin-chat-export"
     />
   </template>

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

@@ -14,6 +14,7 @@
         @title="chat.incoming_webhooks.new"
         @route="adminPlugins.show.discourse-chat-incoming-webhooks.new"
         @routeModels="chat"
+        @icon="plus"
         class="admin-incoming-webhooks-new"
       />
     </:actions>