Move site content between installations as a package

System now exports a content package and imports one, by upload or from
var/pakete. Pages with their revisions, areas, menus, media, forms,
redirects, terms, translations, design and settings are replaced;
submissions, statistics, tasks, plugin state and service keys stay, and
accounts only come along when that is chosen explicitly. A backup comes
first, the site is in maintenance meanwhile, and every file and document
in the package is checked before anything is written.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

config/services.yaml

@@ -118,6 +118,10 @@ services:
             $root: '%kernel.project_dir%'
             $databaseUrl: '%env(projectdir:DATABASE_URL)%'
             $mediaDirectory: '%env(projectdir:MEDIA_DIRECTORY)%'
+    App\Ops\SitePackage:
+        arguments:
+            $root: '%kernel.project_dir%'
+            $mediaDirectory: '%env(projectdir:MEDIA_DIRECTORY)%'
     # Der Aufgabenplan. Die Aufgaben kommen ueber ihr Etikett herein, damit diese Stelle nicht
     # jedes Mal mitwaechst.
     App\Ops\Scheduler: